The role of membrane estrogen receptor-α (mERα) in rapid nongenomic responses to 17β-estradiol (E₂) was tested in sublines of GH3/B6 rat prolactinoma cells selected for high (GH3/B6/F10) and low (GH3/B6/D9) mERα expression. E₂ elicited rapid, concentration-dependent intracellular Ca²⁺ concentration ([Ca²⁺]_i) increases in the F10 subline. Lack of inhibition by thapsigargin depletion of intracellular Ca²⁺ pools, together with abrogation of the response in Ca²⁺-free medium, suggested an extracellular source of Ca²⁺ for this response. The participation of voltage-dependant channels in the E₂-induced [Ca²⁺]_i increase was confirmed by the specific L-type Ca²⁺ channel inhibitor nifedipine. For comparison, the D9 mERα-depleted subline was insensitive to steroid action via this signaling mechanism. [Ca²⁺]_i elevation was correlated with prolactin (PRL) release in the F10 cell line in as little as 3 min. E₂ caused a much higher PRL release than KCl treatment (which caused maximal Ca²⁺ elevation), suggesting that secretion was also controlled by additional mechanisms. Participation of mERα in these effects was confirmed by the ability of E₂-peroxidase (a cell-impermeable analog of E₂) to cause these responses, blockage of the responses with the ER antagonist ICI 182 780, and the inability of the E₂ stereoisomer 17α-E₂ to elicit a response. Thus rapid exocytosis of PRL is regulated in these cells by mERα signaling to specific Ca²⁺ channels utilizing extracellular Ca²⁺ sources and additional signaling mechanisms.