Fifty-four 26-day-old female rats of the CD strain were divided into four groups of 10 or 11 each, i.e. 1) controls; 2) animals given 0.3 ml of a protein-free bovine pineal extract daily intraperitoneally for 28 days; 3) sham-operated animals; 4) pinealectomized animals. In a second experiment two groups of six each were divided into a) control animals given a diet restricted with respect to Na, K and phosphate, and b) animals on the same diet, given 1.0 ml/day of the pineal extract. Pinealectomy caused ovarian hypertrophy (P < .001), pituitary hypertrophy (P < .001), and adrenal hypertrophy (P < .05). The extract reversed these changes. Giving the extract to intact animals caused effects varying with the dose. The dose of 0.3 ml given daily caused gonadal atrophy (P < .05), pituitary atrophy (P < .001) and insignificant adrenal activity. The dose of 1.0 ml caused further gonadal atrophy (P < .001) and significant adrenal atrophy (P < .05).