The aim of the present study was to ascertain whether sex differences exist in GH response to GHRH, and the influence that menstrual cycle have on this response. A GHRH test was performed on nine healthy men and ten women on days one and twelve of the menstrual cycle. Basal GH levels, (mean +/- SE) (7.98 +/- 3.09 ng/ml in women and 0.13 +/- 0.07 ng/ml in men, p less than 0.05) as well as maximal GH response (40.17 +/- 11.96 ng/ml in women and 9.63 +/- 2.32 ng/ml in males, p less than 0.01) were significantly higher in women than in men. In spite of a significant increase in estradiol levels during the menstrual cycle (75.88 +/- 2.48 pg/ml on day one and 198.40 +/- 28.65 pg/ml on day twelve, p less than 0.01) neither basal plasma GH levels, nor GH response to GHRH were significantly different. In conclusion, these results confirm that GH response to GHRH is higher in women than in men, and that this difference is not modified during the first phase of the menstrual cycle.