35 women volunteered to participate in a preliminary study of the residual activity of the ovaries under low dose oral contraceptives. The women were aged 17-35 years and were in good health. 23 took low dose triphasic pills, 10 took low dose monophasic pills, and 2 took higher dose monophasic pills. 101 sonographic examinations of the ovaries were conducted. 5 patients had 5 sonograms during the same cycle. Most initial sonograms were done on about the 12th cycle day, with a repeat examination 2-7 days later if a structure suggestive of a follicle was detected. Radioimmunologic studies of 2 pituitary hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH), were conducted to evaluate the level of pituitary inhibition. Levels of 2 or 3 steroid hormones (estradiol, estrone, and progesterone in the later stages of the cycle) were measured. 56 blood samples for the hormone studies were obtained from 28 of the women. 25 of the 50 observed cycles showed no evidence of follicular structures. In 19 cycles a possible follicle measuring less than 10 mm in diameter was observed and in 10 a possible follicle measured over 10 mm. In only 1 case a possible follicle attained a "preovulatory" diameter of 20 mm on the 13th day. It continued to grow to 31 mm on the 24th day and was slightly smaller on the 28th day. By the 5th day of the following cycle it had disappeared. This patient also had significantly elevated estrogen levels. Significant release of pituitary gonadotropins was observed in 20 cycles at about the 12th day, with values declining around the 21st day in the cycles observed. It can be inferred that low dose pills permit significant release of pituitary gonadotropins in 40% of cases, but preovulatory peak levels are never attained. In all cases the level of progesterone was at the limit of detectability.