Synthetic hGHRH was incubated with dispersed pituitary cells from 14 human fetuses at 14–23 weeks of fetal age. After at least three days following plating the cells on an extracellular matrix in serum-containing medium, 3 h incubation with hGHRH in serum-free medium induced a significant increase in GH secretion into the medium at concentrations of hGHRH of 0.01 nmol/1 or higher. After 24 h exposure to 0.5 nmol/1 hGHRH, subsequent incubation with 0.5 nmol/1 hGHRH for 3 h induced significantly lower GH secretion into the medium compared to the GH secretion by cells exposed previously to medium alone. In contrast, when the subsequent exposure to hGHRH was at 10-fold higher concentrations than the concentration present in the initial exposure, GH secretion into the medium did not significantly decrease compared to previously untreated cells. These results suggest that desensitization (down-regulation) of GHRH receptors on somatotropes may be involved in the mechanism by which prior GHRH exposure inhibits GH secretion in response to subsequent GHRH administration. Such desensitization seems to occur in the human fetus but is incomplete. The desensitization may be overcome by increasing the GHRH concentrations with subsequent exposure, suggesting that cellular GH depletion is not responsible for the decreased responsiveness observed.