The origin of pulsatile LH secretion and its modulation by hypothalamic catecholamines were studied in long-term ovariectomized rats. When the medial basal hypothalamus (MBH) was completely deafferentated, the suprachiasmatic nuclei being excluded from the island, episodic LH secretion with a lowered basal level persisted, and periods were unchanged. When the pituitary portal vessels were effectively lesioned electrically, pulsatile fluctuation in the plasma LH level was abolished with a lowered basal level. Various doses (from 10(-3) to 10(-6) M) of dopamine (DA), apomorphine (APO) and norepinephrine (NE) were injected into the lateral ventricle of the brain of ovariectomized rats. After injection of DA and APO separately, episodic LH secretion was markedly suppressed and LH levels were lowered. Similar effects were obtained after intraventricular injection of NE. When brain catecholamines were depleted by prior treatment with 6-hydroxydopamine (6-OHDA), basal levels in ovariectomized rats were elevated and the amplitude was augmented without noticeable change in the frequency of LH episodes. It is surmised that LH episodism originates in the MBH and both DA and NE exert an inhibitory action on it. The mode of action of the catecholamines appears not frequency-modulated, but of amplitude modulation of LH episodism.