diff --git a/src/hll/Math.c b/src/hll/Math.c
index e6a263a..d3e07c8 100644
--- a/src/hll/Math.c
+++ b/src/hll/Math.c
@@ -23,9 +23,23 @@
 #define M_PI (3.14159265358979323846)
 #endif
 
+#include "system4/mt19937int.h"
+
 #include "hll.h"
 #include "vm/page.h"
 
+static struct mt19937 mt;
+static bool mt_initialized;
+
+static uint32_t math_genrand(void)
+{
+	if (!mt_initialized) {
+		mt19937_init(&mt, 4357);
+		mt_initialized = true;
+	}
+	return mt19937_genrand(&mt);
+}
+
 struct shuffle_table {
 	int id;
 	struct shuffle_table *next;
@@ -64,9 +78,16 @@ static float Math_Tan(float x)
 	return tanf(deg2rad(x));
 }
 
+static void Math_SetSeed(int seed)
+{
+	mt19937_init(&mt, (uint32_t)seed | 1);
+	mt19937_genrand(&mt);  // the original engine discards the first value
+	mt_initialized = true;
+}
+
 static void Math_SetSeedByCurrentTime(void)
 {
-	srand(time(NULL));
+	Math_SetSeed(time(NULL));
 }
 
 static int Math_Min(int a, int b)
@@ -105,9 +126,14 @@ static void Math_SwapF(float *a, float *b)
 
 //void Math_SetRandMode(int mode);
 
+static int Math_Rand(void)
+{
+	return math_genrand() & 0x7fffffff;
+}
+
 static float Math_RandF(void)
 {
-	return rand() * (1.0 / (RAND_MAX + 1U));
+	return (float)(math_genrand() * (1.0 / 4294967296.0));
 }
 
 static void shuffle_array(int *a, int len)
@@ -192,10 +218,10 @@ HLL_LIBRARY(Math,
 	    HLL_EXPORT(Abs, abs),
 	    HLL_EXPORT(AbsF, fabsf),
 	    HLL_EXPORT(Pow, powf),
-	    HLL_EXPORT(SetSeed, srand),
+	    HLL_EXPORT(SetSeed, Math_SetSeed),
 	    HLL_EXPORT(SetSeedByCurrentTime, Math_SetSeedByCurrentTime),
 	    //HLL_EXPORT(SetRandMode, Math_SetRandMode),
-	    HLL_EXPORT(Rand, rand),
+	    HLL_EXPORT(Rand, Math_Rand),
 	    HLL_EXPORT(RandF, Math_RandF),
 	    HLL_EXPORT(RandTableInit, Math_RandTableInit),
 	    HLL_EXPORT(RandTable, Math_RandTable),
diff --git a/test/Run/test.ain b/test/Run/test.ain
index 77fd2ee..af7afe5 100644
Binary files a/test/Run/test.ain and b/test/Run/test.ain differ
diff --git a/test/Source/math.jaf b/test/Source/math.jaf
index 7db4057..ec903db 100755
--- a/test/Source/math.jaf
+++ b/test/Source/math.jaf
@@ -31,6 +31,25 @@ void test_math(void)
 	test_float("Math.SwapF(a, b), a", fa, 2.0);
 	test_float("Math.SwapF(a, b), b", fb, 1.0);
 
+	// The default seed is 4357. This must run before any other random
+	// number is drawn.
+	test_equal("Math.Rand() #1 (default seed)", Math.Rand(), 1362922229);
+	test_equal("Math.Rand() #2 (default seed)", Math.Rand(), 2143450242);
+
+	Math.SetSeed(12345);
+	test_equal("Math.Rand() #1 (seed 12345)", Math.Rand(), 2120104832);
+	test_equal("Math.Rand() #2 (seed 12345)", Math.Rand(), 377474682);
+
+	// SetSeed ORs the seed with 1.
+	Math.SetSeed(12344);
+	test_equal("Math.Rand() #1 (seed 12344)", Math.Rand(), 2120104832);
+	test_equal("Math.Rand() #2 (seed 12344)", Math.Rand(), 377474682);
+
+	Math.SetSeed(12345);
+	test_float("Math.RandF() #1", Math.RandF(), 0.993625);
+	test_float("Math.RandF() #2", Math.RandF(), 0.587888);
+	test_float("Math.RandF() #3", Math.RandF(), 0.857469);
+
 	Math.RandTableInit(42, 10);
 	for (i = 0; i < 10; i++) {
 		ar[Math.RandTable(42)] = 1;