Administration of 17β-estradiol to ovariectomized rats increased the concentrations of galanin-like immunoreactivity (LI) in the hippocampal formation by 215% (P < 0.001) within 1 h. An increase of 125% (P < 0.05) was observed in the same brain region in the proestrous phase of a normal estrous cycle. Tamoxifen® did not block the 17β-estradiol-induced increase in the concentration of galanin-LI but resulted in a 62% decrease in the hypothalamus within 1 h. In vivo microdialysis in the dorsal hippocampal formation showed a decrease of extracellular galanin-LI (P < 0.001) 1−2 h after treatment with 17β-estradiol, indicating a decreased release of galanin. For comparision, we studied the concentrations of neuropeptide Y, which were not influenced significantly in any of the regions studied. Taken together our results suggest that 17β-estradiol inhibits galanin release, presumably from noradrenergic nerve terminals, and primarily via a nongenomic/indirect action, not necessarily involving the classical nuclear receptors ER-α or ER-β. These rapid estrogen-induced changes in galanin release could influence transmitter signalling and plasticity in the hippocampal formation.