The effects of gamma-aminobutyric acid (GABA) increase at the nucleus locus coeruleus (LC) on pro-oestrus LH release and ovulation were evaluated in rats. Local microinjection of the GABA-transaminase inhibitor gamma-vinyl-GABA (GVG) produced 6 h later a marked increase in GABA in the LC. Such action caused a significant decrease of plasma LH levels and prevented the pro-oestrus LH surge. In some animals, plasma prolactin levels were also lowered, but in others its plasma concentrations were high and similar to that in controls. Ovulation did not occur in the rats treated with GVG. In additional experiments, the periventricular gray substance (PGS) close to the locus coeruleus was injected with GVG. Results obtained show a LH surge blockade and failure of ovulation in most of these rats. These findings may be interpreted on the basis of GABA action on rostral LC cells that project to the PGS. The results altogether suggest that through neurons of the locus coeruleus, GABA may exert an inhibitory role in the regulation of LH secretion.