diff --git a/middleware.go b/middleware.go
index 162342a5..333be41d 100644
--- a/middleware.go
+++ b/middleware.go
@@ -116,28 +116,21 @@ func parseRequestURL(c *Client, r *Request) error {
 }
 
 func parseRequestHeader(c *Client, r *Request) error {
-	hdr := make(http.Header)
-	for k := range c.Header {
-		hdr[k] = append(hdr[k], c.Header[k]...)
-	}
-
-	for k := range r.Header {
-		hdr.Del(k)
-		hdr[k] = append(hdr[k], r.Header[k]...)
+	for k, v := range c.Header {
+		if _, ok := r.Header[k]; ok {
+			continue
+		}
+		r.Header[k] = v[:]
 	}
 
-	if IsStringEmpty(hdr.Get(hdrUserAgentKey)) {
-		hdr.Set(hdrUserAgentKey, hdrUserAgentValue)
+	if IsStringEmpty(r.Header.Get(hdrUserAgentKey)) {
+		r.Header.Set(hdrUserAgentKey, hdrUserAgentValue)
 	}
 
-	ct := hdr.Get(hdrContentTypeKey)
-	if IsStringEmpty(hdr.Get(hdrAcceptKey)) && !IsStringEmpty(ct) &&
-		(IsJSONType(ct) || IsXMLType(ct)) {
-		hdr.Set(hdrAcceptKey, hdr.Get(hdrContentTypeKey))
+	if ct := r.Header.Get(hdrContentTypeKey); IsStringEmpty(r.Header.Get(hdrAcceptKey)) && !IsStringEmpty(ct) && (IsJSONType(ct) || IsXMLType(ct)) {
+		r.Header.Set(hdrAcceptKey, r.Header.Get(hdrContentTypeKey))
 	}
 
-	r.Header = hdr
-
 	return nil
 }
 
diff --git a/middleware_test.go b/middleware_test.go
index fef6f008..a9bac8c7 100644
--- a/middleware_test.go
+++ b/middleware_test.go
@@ -1,7 +1,9 @@
 package resty
 
 import (
+	"net/http"
 	"net/url"
+	"reflect"
 	"testing"
 )
 
@@ -227,3 +229,117 @@ func Test_parseRequestURL(t *testing.T) {
 		})
 	}
 }
+
+func Test_parseRequestHeader(t *testing.T) {
+	for _, tt := range []struct {
+		name           string
+		init           func(c *Client, r *Request)
+		expectedHeader http.Header
+	}{
+		{
+			name: "headers in request",
+			init: func(c *Client, r *Request) {
+				r.SetHeaders(map[string]string{
+					"foo": "1",
+					"bar": "2",
+				})
+			},
+			expectedHeader: http.Header{
+				http.CanonicalHeaderKey("foo"):           []string{"1"},
+				http.CanonicalHeaderKey("bar"):           []string{"2"},
+				http.CanonicalHeaderKey(hdrUserAgentKey): []string{hdrUserAgentValue},
+			},
+		},
+		{
+			name: "headers in client",
+			init: func(c *Client, r *Request) {
+				c.SetHeaders(map[string]string{
+					"foo": "1",
+					"bar": "2",
+				})
+			},
+			expectedHeader: http.Header{
+				http.CanonicalHeaderKey("foo"):           []string{"1"},
+				http.CanonicalHeaderKey("bar"):           []string{"2"},
+				http.CanonicalHeaderKey(hdrUserAgentKey): []string{hdrUserAgentValue},
+			},
+		},
+		{
+			name: "headers in client and request",
+			init: func(c *Client, r *Request) {
+				c.SetHeaders(map[string]string{
+					"foo": "1", // ignored, because of the same header in the request
+					"bar": "2",
+				})
+				r.SetHeaders(map[string]string{
+					"foo": "3",
+					"xyz": "4",
+				})
+			},
+			expectedHeader: http.Header{
+				http.CanonicalHeaderKey("foo"):           []string{"3"},
+				http.CanonicalHeaderKey("bar"):           []string{"2"},
+				http.CanonicalHeaderKey("xyz"):           []string{"4"},
+				http.CanonicalHeaderKey(hdrUserAgentKey): []string{hdrUserAgentValue},
+			},
+		},
+		{
+			name: "no headers",
+			init: func(c *Client, r *Request) {},
+			expectedHeader: http.Header{
+				http.CanonicalHeaderKey(hdrUserAgentKey): []string{hdrUserAgentValue},
+			},
+		},
+		{
+			name: "user agent",
+			init: func(c *Client, r *Request) {
+				c.SetHeader(hdrUserAgentKey, "foo bar")
+			},
+			expectedHeader: http.Header{
+				http.CanonicalHeaderKey(hdrUserAgentKey): []string{"foo bar"},
+			},
+		},
+		{
+			name: "json content type",
+			init: func(c *Client, r *Request) {
+				c.SetHeader(hdrContentTypeKey, "application/json")
+			},
+			expectedHeader: http.Header{
+				http.CanonicalHeaderKey(hdrContentTypeKey): []string{"application/json"},
+				http.CanonicalHeaderKey(hdrAcceptKey):      []string{"application/json"},
+				http.CanonicalHeaderKey(hdrUserAgentKey):   []string{hdrUserAgentValue},
+			},
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			c := New()
+			r := c.R()
+			tt.init(c, r)
+			if err := parseRequestHeader(c, r); err != nil {
+				t.Errorf("parseRequestHeader() error = %v", err)
+			}
+			if !reflect.DeepEqual(tt.expectedHeader, r.Header) {
+				t.Errorf("r.Header = %#+v does not match expected %#+v", r.Header, tt.expectedHeader)
+			}
+		})
+	}
+}
+
+func Benchmark_parseRequestHeader(b *testing.B) {
+	c := New()
+	r := c.R()
+	c.SetHeaders(map[string]string{
+		"foo": "1", // ignored, because of the same header in the request
+		"bar": "2",
+	})
+	r.SetHeaders(map[string]string{
+		"foo": "3",
+		"xyz": "4",
+	})
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if err := parseRequestHeader(c, r); err != nil {
+			b.Errorf("parseRequestHeader() error = %v", err)
+		}
+	}
+}