Addition of LHRH for 40 h to aggregate cell cultures of 14-day-old rat pituitary significantly decreased the number of [3H]thymidine ([3H]T)-incorporating cells which immunostained for GH protein as well as the number of [3H]T-labelled cells expressing GH mRNA detectable by in situ hybridization with a digoxigenin-labelled riboprobe. The effect at the level of GH protein was seen at a dose of 1 nM LHRH. However, the effect at the GH mRNA level required a higher dose of LHRH (10 nM) or a longer incubation time (64 h). Treatment of the cultures for 40 h with 0.1 nM GH-releasing factor (GRF) provoked a 54% increase in the number of [3H]T-labelled cells containing GH mRNA and a 30% increase in the number of cells immunostained for GH protein. The latter effects of GRF were completely blocked by simultaneous addition of LHRH (1 nM) to the cultures. In the absence of GRF, LHRH (1 nM) also had an inhibitory effect on the total number of cells containing GH mRNA and a comparable effect on the total number of cells stained for GH protein. The present data show that LHRH is capable of inhibiting the GRF-independent as well as the GRF-dependent development of somatotrophs in postnatal rat pituitary in culture.