The involvement of γ-aminobutyric acid (GABA) in the control of prolactin (PRL) release was investigated in rainbow trout using both perifused pituitary fragments and pituitary cells in primary culture. In our perifusion system, infusion of GABA (10(-6) to 10(-4) M) caused an inhibition of PRL release (between 20 and 40%). Administration on perifused pituitary fragments of 3APS, a GABAa agonist, mimicked this inhibitory effect. Moreover, bicuculline, a specific antagonist of GABAa receptors, totally abolished GABA effect. When tested on cultured pituitary cells during 40h exposure, GABA (10(-5) M) caused a significant decrease in PRL release (24.5%). Baclofen, a specific agonist for GABAb receptor tested at 10(-6) and 10(-5) M, also inhibited PRL released from cultured pituitary cells. These results demonstrate that GABA inhibits PRL release by acting directly on pituitary cells and that probably both types of GABA receptor (a and b) are involved in this regulation.