We have examined the effects of cholera toxin, a specific probe for processes which are caused by an increase in cAMP, on clonal strains of rat pituitary cells (GH cells) in culture. We found that 5 ng/ml cholera toxin increased the amount of intracellular cAMP after a lag period of 30-60 min. After a similar lag period, cholera toxin increased the release of PRL into the medium by 50% and caused a decrease in intracellular PRL to 60% of control values at 90 min. PRL synthesis was increased to 100% above control after 1 week of treatment, and GH synthesis was elevated to 280% above control. TRH caused an increase in PRL release and synthesis but a decrease of GH synthesis. Depending on the dose, TRH prevented or reduced the cholera toxin-induced rise in GH synthesis. We conclude that the decrease in GH synthesis caused by TRH is not likely to be mediated through an increase of intracellular cAMP.