The effects of a series of catecholestrogens (2-hydroxyestrogens and 4-hydroxyestrogens) were compared to those of a primary estrogen, ethynylestradiol (EE2), on the catecholaminergic system in the preoptic-hypothalamic rat brain. Adult ovariectomized rats received single injections (100 microgram s.c.) of EE2, 2-hydroxyestradiol (2-OHE2), 2-hydroxyethynylestradiol (2-OHEE2), 4-hydroxyestradiol (4-OHE2) or 4-hydroxyethynylestradiol (4-OHEE2). Eight hours after estrogen administration the animals were killed, and the concentrations of dopamine (DA), noradrenaline (NA) and adrenaline (A) were determined radioenzymatically in the preoptic area (POA) and the mediobasal hypothalamus (MBH). EE2 and all the catecholestrogens tested uniformly suppressed (P less than 0.01) serum LH levels. This correlated well with the decreased turnover rate of A in the POA. In both the POA and the MBH of EE2-treated animals the turnover rate of NA was markedly decreased whereas the concentrations of catecholamines remained unaffected. The catecholethynylestrogens, potent inhibitors of catechol O-methyltransferase, caused two-fold increases of NA concentrations in the POA (2-OHEE2 and 4-OHEE2) and MBH (2-OHEE2) without affecting the turnover rate. Thus, although EE2 and all the catecholestrogens tested uniformly suppressed LH release they induced highly different effects on the noradrenergic system In the preoptic-hypothalamic brain.