To demonstrate the effect of diazepam on serum growth hormone and prolactin levels in acromegaly, 7 acromegalics were studied. Every patient received 10 mg diazepam orally at zero time, and blood samples were obtained at 30-min intervals. The same patients also underwent a standard L-dopa test. They were given 500 mg L-dopa orally at zero time, and blood samples were obtained at 30-min intervals. The administration of diazepam did not induce any significant changes in either growth hormone or prolactin levels. On the other hand, the administration of L-dopa resulted in a significant decrease in growth hormone level from 74.25 ± 22 ng/ml (mean ± SEM) at zero time to a level of 52.8 ± 21 and 58.77 ± 22.7 ng/ml at the 60- and 90-min intervals, respectively (p < 0.05), and a significant decrease in prolactin level from a baseline of 56.18 ± 17 to 25.5 ± 8.4 ng/ml (p < 0.001) at the 60-min interval.It is suggested that the response of growth hormone to diazepam in acromegalics is qualitatively different from nonacromegalic controls.