The effect of gamma-aminobutyric acid (GABA) on levels of messenger ribonucleic acid (mRNA) encoding prolactin (PRL) was studied in cultured anterior pituitary cells, in vitro and in intact rats, in vivo. As quantitated by hybridization to a 32P-labeled rat PRL complementary deoxyribonucleic acid (cDNA) probe, levels of PRL mRNA in cultured pituitary cells were decreased by about 50% following 3 days exposure to 10(-5) M GABA. This effect was mimicked by muscimol (10(-6) M) and antagonized by bicuculline (10(-5) M). An increase of endogenous GABA levels in vivo effected by injection of GABA transaminase blockers (aminooxyacetic acid, 20 mg/kg, twice daily; vinyl GABA, 800 mg/kg) into rats resulted in a similar decrease in rat PRL mRNA levels in the adenohypophysis 3-4 days following commencement of the drug treatment. These findings suggest that GABA might inhibit PRL gene expression by a direct action on lactotrophs of the adenohypophysis.