Receptors for calcitriol are described in human decidua. They exhibit a dissociation constant of 35 +/- 6 pM and they are at concentrations similar to those found in other tissues (0.45 +/- 0.04 fmol/micrograms DNA). They are highly specific for calcitriol since neither of the other vitamin D3 derivatives nor the steroid hormone tested displaced labeled calcitriol from the receptor. Also, calcitriol at concentrations of 10(-13) to 10(-11) M stimulates prolactin secretion by dispersed decidual cells. At these concentrations, however, the hormone has no effect on prostaglandin production. The specificity of calcitriol action was further examined by studying the effect of estrogen, progesterone, testosterone, dexamethasone and cortisol, all at 10(-7) M, on the secretion of prolactin and prostaglandins. Under the conditions used in this study, the steroids have no effect on prolactin secretion; but dexamethasone significantly inhibits prostaglandin F2 alpha output by the cells. Taken together with previous studies from our laboratory demonstrating that decidua can synthesize calcitriol the present study indicates that this hormone has an autocrine effect on human decidual cells.