To clarify the role and site of action of γ-aminobutyric acid (GABA) in ovine pineal glands, we have investigated the effects of aminooxyacetic acid (AOAA), an inhibitor of GABA transaminase, on endogenous GABA content and β-adrenoceptor mediated pineal function in Merino sheep. A significant elevation of endogenous GABA levels was noted in the glands, but no effect was observed on radioligand binding in vitro to pineal β-adrenoceptors following in vivo administration of AOAA. Incubation of washed pineal membranes with GABA or AOAA had no effect on ligand binding to β-adrenoceptors. Incubation of Merino pineal slices with GABA inhibited isoprenaline-stimulated but not basal serotonin N-acetyltransferase (NAT) activity. Incubation of whole pineal homogenates with GABA was without effect on either isoprenaline-stimulated or basal adenyl cyclase activity. Thus, Merino pineal glands resemble bovine pineals in that β-adrenoceptor mediated melatonin biosynthesis in both species may be regulated in part by GABA. Our results indicate that GABA may exert its effect on Merino pineal NAT activity at a locus distal to the site of action of adenyl cyclase; however, the detailed mechanism and physiological role of this regulation remain to be elucidated.