1 Orally administered glyceryl trinitrate to rats undergoes extensive first pass metabolism leading to low bioavailability. 2 Sex differences in the plasma elimination of glyceryl trinitrate were seen in the rat, the female exhibiting the longer plasma half-life. No sex differences in this respect were detected in the rabbit. 3 The plasma half-life of glyceryl trinitrate was longer and the volume of distribution larger, in older animals. 4 The plasma elimination of glyceryl trinitrate was different in various animal species. There was a good correlation between plasma half-life and animal bodyweight.