1. 1. Experiments were conducted to investigate the response of plasma growth hormone (GH) to seasonal temperature and temperature acclimation, both individually and in combination with sustained exercise. 2. 2. The effect of varying the intensity and duration of sustained exercise on plasma GH levels was also assessed. 3. 3. Seasonal GH levels were lowest at the cold temperature of the winter months and highest at peak summer temperatures. 4. 4. Exercise at 1.5 bl/sec for a 24 hr period at high temperature caused a greater increase in plasma GH levels than the same exercise at low temperature. 5. 5. For exercise bouts of similar total work, plasma GH response was greater in the high intensity, shorter duration bout than for the low intensity, long duration bout. Exhaustive exercise resulted in no change in GH levels post-exercise.