--- Objects/stringobject.c
+++ Objects/stringobject.c
@@ -1975,10 +1975,14 @@
 \n\
 Return a copy of the string S converted to lowercase.");
 
-/* _tolower and _toupper are defined by SUSv2, but they're not ISO C */
-#ifndef _tolower
-#define _tolower tolower
-#endif
+static int
+tolower_C_locale(int c)
+{
+    if (c >= 'A' && c <= 'Z')
+        return c + 0x20;
+    else
+        return c;
+}
 
 static PyObject *
 string_lower(PyStringObject *self)
@@ -1998,7 +2002,7 @@
     for (i = 0; i < n; i++) {
         int c = Py_CHARMASK(s[i]);
         if (isupper(c))
-            s[i] = _tolower(c);
+            s[i] = tolower_C_locale(c);
     }
 
     return newobj;
@@ -2009,9 +2013,14 @@
 \n\
 Return a copy of the string S converted to uppercase.");
 
-#ifndef _toupper
-#define _toupper toupper
-#endif
+static int
+toupper_C_locale(int c)
+{
+    if (c >= 'a' && c <= 'z')
+        return c - 0x20;
+    else
+        return c;
+}
 
 static PyObject *
 string_upper(PyStringObject *self)
@@ -2031,7 +2040,7 @@
     for (i = 0; i < n; i++) {
         int c = Py_CHARMASK(s[i]);
         if (islower(c))
-            s[i] = _toupper(c);
+            s[i] = toupper_C_locale(c);
     }
 
     return newobj;
@@ -2059,11 +2068,11 @@
         int c = Py_CHARMASK(*s++);
         if (islower(c)) {
             if (!previous_is_cased)
-                c = toupper(c);
+                c = toupper_C_locale(c);
             previous_is_cased = 1;
         } else if (isupper(c)) {
             if (previous_is_cased)
-                c = tolower(c);
+                c = tolower_C_locale(c);
             previous_is_cased = 1;
         } else
             previous_is_cased = 0;
@@ -2092,7 +2101,7 @@
     if (0 < n) {
         int c = Py_CHARMASK(*s++);
         if (islower(c))
-            *s_new = toupper(c);
+            *s_new = toupper_C_locale(c);
         else
             *s_new = c;
         s_new++;
@@ -2100,7 +2109,7 @@
     for (i = 1; i < n; i++) {
         int c = Py_CHARMASK(*s++);
         if (isupper(c))
-            *s_new = tolower(c);
+            *s_new = tolower_C_locale(c);
         else
             *s_new = c;
         s_new++;
@@ -2171,10 +2180,10 @@
     for (i = 0; i < n; i++) {
         int c = Py_CHARMASK(*s++);
         if (islower(c)) {
-            *s_new = toupper(c);
+            *s_new = toupper_C_locale(c);
         }
         else if (isupper(c)) {
-            *s_new = tolower(c);
+            *s_new = tolower_C_locale(c);
         }
         else
             *s_new = c;