diff --git a/rules/lexer.l b/rules/lexer.l
index 1edfcc6b09..4904f7a5b3 100644
--- a/rules/lexer.l
+++ b/rules/lexer.l
@@ -17,6 +17,7 @@ package rules
 import (
         "github.com/prometheus/prometheus/model"
         "strconv"
+        "strings"
 )
 %}
 
@@ -39,6 +40,7 @@ U                       [smhdwy]
 permanent                { return PERMANENT }
 BY                       { return GROUP_OP }
 AVG|SUM|MAX|MIN          { yylval.str = yytext; return AGGR_OP }
+avg|sum|max|min          { yylval.str = strings.ToUpper(yytext); return AGGR_OP }
 \<|>|AND|OR              { yylval.str = yytext; return CMP_OP }
 ==|!=|>=|<=              { yylval.str = yytext; return CMP_OP }
 [+\-]                    { yylval.str = yytext; return ADDITIVE_OP }
diff --git a/rules/lexer.l.go b/rules/lexer.l.go
index 0671c45afb..8f66be6108 100644
--- a/rules/lexer.l.go
+++ b/rules/lexer.l.go
@@ -12,6 +12,7 @@ import (
 import (
 "github.com/prometheus/prometheus/model"
 "strconv"
+"strings"
 )
 
 var yyin io.Reader = os.Stdin
@@ -341,6 +342,20 @@ var yyrules []yyrule = []yyrule{{regexp.MustCompile("[^\\n]"), nil, []yystartcon
 		return yyactionreturn{AGGR_OP, yyRT_USER_RETURN}
 	}
 	return yyactionreturn{0, yyRT_FALLTHROUGH}
+}}, {regexp.MustCompile("avg|sum|max|min"), nil, []yystartcondition{}, false, func() (yyar yyactionreturn) {
+	defer func() {
+		if r := recover(); r != nil {
+			if r != "yyREJECT" {
+				panic(r)
+			}
+			yyar.returnType = yyRT_REJECT
+		}
+	}()
+	{
+		yylval.str = strings.ToUpper(yytext)
+		return yyactionreturn{AGGR_OP, yyRT_USER_RETURN}
+	}
+	return yyactionreturn{0, yyRT_FALLTHROUGH}
 }}, {regexp.MustCompile("\\<|>|AND|OR"), nil, []yystartcondition{}, false, func() (yyar yyactionreturn) {
 	defer func() {
 		if r := recover(); r != nil {
diff --git a/rules/rules_test.go b/rules/rules_test.go
index a972de4e21..68a366abe9 100644
--- a/rules/rules_test.go
+++ b/rules/rules_test.go
@@ -210,20 +210,29 @@ var expressionTests = []struct {
 		fullRanges:     0,
 		intervalRanges: 8,
 	}, {
+		// Single-letter label names and values.
 		expr: "x{y='testvalue'}",
 		output: []string{
 			"x{y='testvalue'} => 100 @[%v]",
 		},
 		fullRanges:     0,
 		intervalRanges: 1,
-		// Invalid expressions that should fail to parse.
 	}, {
+		// Lower-cased aggregation operators should work too.
+		expr:           "sum(http_requests) + min(http_requests) + max(http_requests) + avg(http_requests)",
+		output:         []string{"http_requests{} => 4950 @[%v]"},
+		fullRanges:     0,
+		intervalRanges: 8,
+	}, {
+		// Empty expressions shouldn't parse.
 		expr:       "",
 		shouldFail: true,
 	}, {
+		// Subtracting a vector from a scalar is not supported.
 		expr:       "1 - http_requests",
 		shouldFail: true,
 	}, {
+		// Interval durations can't be in quotes.
 		expr:       "http_requests['1m']",
 		shouldFail: true,
 	},