Controversy over effects of oral contraceptives (OCs) on serum prolactin (PRL) levels from retrospective studies suggested performing a prospective study. Statistical analyses of PRL levels in 552 reproductive-age, nonmedicated women indicated a provisionally lognormal distribution of values less than 15 ng/ml, contaminated by a small number of abnormally high values less than or equal to 90 mg/ml. Truncated samples were used to estimate a "normal range" of PRL levels for three subsets of the study sample, classified according to number of weeks after pregnancy. Fifty-microgram estrogen-containing OCs doubled basal PRL levels at 5 to 8 weeks in those whose initial control values fell below 15 ng/ml, but the PRL elevation was no longer evident at 6 months of drug use. These OCs induced a small but significant lowering of PRL at 5 to 8 weeks in those with control levels of 15 ng/ml or higher. Thirty-five-microgram estrogen-containing OCs failed to alter PRL levels at 5 to 8 weeks in those with control values less than 15 ng/ml.