The influence of GABA on the synthesis of N-acetylserotonin, melatonin, O-acetyl-5-hydroxytryptophol and O-acetyl-5-methoxytryptophol has been investigated using different experimental procedures. It was demonstrated that when GABA and an acetyl donor were added to the incubation medium together, a significant increase in synthesis of the N-acetylated products occurred during the night. Moreover there was a large increase in N-acetylserotonin synthesis at 15(00) hrs although none was observed in the control experiments. However, when GABA was added 20 min before the acetyl donor, synthesis of the N-acetylated products was significantly less. The opposite effect was observed for the O-acetylated indoles. These results confirm the proposal by Ebadi et al. (1982) that GABA, like norepinephrine, may be a regulator of melatonin synthesis. As melatonin is implicated in the regulation of reproduction it may be that GABA is equally significant in this regulatory effect.