The frequency and amplitude of GnRH and LH pulses are variable and controlled by both external environmental and internal physiological factors. However, the specific neurochemical-neuroanatomical pathways that control the basal pulsatile and surge patterns, and mediate responses to environmental and physiological factors are poorly defined. The gamma-aminobutyric acid (GABA) secretory system is one of several that modulate GnRH and LH. GABA release in the preoptic area (POA) preceding the onset of oestrogen-induced LH surges changes in a pattern that is inverse to LH release. Application of GABA agonists or antagonists to either the POA or mediobasal hypothalamus disrupts LH secretion. Observations that application of GABA(B) agonists to either the POA or mediobasal hypothalamus rapidly reverses the negative feedback effect of oestrogen or testosterone on LH lead the authors to suggest that GABA(B) receptors have an important role in regulating LH secretion in sheep.