17β‐estradiol and 1,25‐dihydroxyvitamin D₃(calcitriol) rapidly increase (< 5 sec) the concentration of intracellular calcium by mobilizing Ca²⁺ from the endoplasmic reticulum and forming inositol 1,4,5‐trisphosphate (InsP₃) and diacylglycerol. Calcitriol increases InsP₃ formation via activation of phospholipase C (PLC)‐β1 linked to a pertussis toxin (PTX)‐insensitive G‐protein, and estradiol via activation of PLC‐β2 linked to a PTX‐sensitive G‐protein. Since PLC are effectors of different subunits of various G‐proteins, we looked for and identified several G‐subunits (Gα_q/11, Gαs, Gαi, Gβ and Gγ) in female rat osteoblasts using Western immunoblotting. The action of calcitriol on InsP₃ formation and Ca²⁺ mobilization in Fura‐2‐loaded confluent osteoblasts involved Gα_q/11. The membrane effects of estradiol involved Gβγ; subunits, and principally Gβ subunits, but not α‐subunits. These results may provide additional evidence for membrane receptors of steroid hormones. Since PLC‐β1 is the target effector of Gα_q/11, whereas PLC‐β2 is only activated by βγ subunits, this specificity may help to generate membrane receptor‐specific responses in vivo. J. Cell. Biochem. 75:138–146, 1999. © 1999 Wiley‐Liss, Inc.