Two experiments were undertaken to study the abilities of clomiphene citrate (20 mg/kg body wt/wk s.c.) and tamoxifen citrate (20 mg/kg body wt/wk s.c.) to slow bone resorption mediated by (a) endogenous parathyroid hormone (PTH) and (b) exogenous calcitriol (1,25(OH)2D3) in vivo in rats with intact ovarian function. Groups of rats with 45Ca-labelled bones were fed a low-calcium (0.01% Ca) diet to stimulate secretion of PTH. Neither clomiphene nor tamoxifen showed the mobilization of 45Ca from femoral bone or prevented the reduction in bone calcium induced by feeding this diet. Moreover these drugs did not depress the urinary excretion of 45Ca or hydroxyproline. These observations indicated that clomiphene and tamoxifen did not inhibit PTH-mediated bone resorption. Administering calcitriol (50 ng/day) orally for 14 days raised plasma calcium, increased urinary 45Ca and its specific activity and decreased femur 45Ca: all these responses were similar in animals receiving calcitriol alone and calcitriol with clomiphene or tamoxifen. The femur 45Ca values (dpm X 10(-3) were: (mean +/- SD, n = 8) placebo, 1901 +/- 127; 1,25(OH)2D3, 1727 +/- 96**; clomiphene + 1,25(OH)2D3, 1694 +/- 93**; tamoxifen + 1,25(OH)2D3, 1664 +/- 61**. (** = P less than 0.01). Thus neither clomiphene nor tamoxifen prevented calcitriol-mediated bone resorption in vivo in the rat.