Abstract. Using the whole-cell patch-clamp technique in freshly isolated rat osteoclasts we examined the effects of estrogen on ionic channels. The predominant current was an inward rectifier K+ current (IKir). In the absence of non-osteoclastic cells, extracellularly applied 17-estradiol (0.1M) inhibited IKir, indicating that estrogen acts directly on osteoclasts. Application of 17-estradiol (10M) for 10min reduced IKir at the membrane potential of 120mV to 7015% of control. Removal of 17-estradiol partially restored the inhibition. The inhibition of IKir was dependent on concentration and application time. Intracellularly applied 17-estradiol had no effect on IKir. 17-Estradiol also inhibited the IKir, whereas progesterone and testosterone had no effect. The inhibitory action of 17-estradiol was not affected by guanosine 5-O-(2-thiodiphosphate) (GDPS), adenosine 3,5-cyclic monophosphothioate Rp diastereomer (Rp-cAMPS), okadaic acid, staurosporine and phorbol ester, and was independent of intracellular Ca2+ concentration ([Ca2+]i). With no influence from soluble factors secreted from non-osteoclastic cells, preincubation of the osteoclastsfor more than 60min with much lower concentrations of 17-estradiol (1 and 10nM) caused a reduction of IKir. In current-clamp configuration, application of 17-estradiol (10M) depolarized the membrane associated with a decrease in a membrane conductance, indicating that 17-estradiol inhibits IKir and depolarizes the membrane of osteoclasts. These results suggest that the 17-estradiol-induced inhibition of IKir might be mediated via non-genomic mechanisms. This direct action of 17-estradiol on osteoclasts may contribute to the regulation of [Ca2+]i and partially account for the protective effects of estrogen against bone loss.