The effects of calcitriol on histomorphometric parameters of bone structure, formation, and resorption were evaluated in 21 normal or oophorectomized rats. Twelve rats were oophorectomized; six of these received daily subcutaneous injections of 135 pmol of calcitriol for 14 wk starting 38 wk after oophorectomy. Nine rats were sham operated; five of these received the same treatment with calcitriol. There were no differences in serum calcium and phosphorus between sham-operated and oophorectomized animals and rats with or without calcitriol. Calcitriol treatment of sham-operated rats produced an increase in cancellous and cortical bone mass. Oophorectomy resulted in decreased cancellous and cortical bone mass and in a decreased ratio between mineralized and nonmineralized bone. There was no difference in bone-osteoclast interface and osteoid seam width among all groups of rats. Calcitriol corrected, at least in part, the bone loss in oophorectomized animals, normalized the ratio between mineralized and nonmineralized bone, and restored mean growth plate width to normal.