PIP: The effect of oral contraceptives on serum concentrations of prolactin and the main ovarian steroids (estradiol and progesterone) was determined. Sera were assessed by radioimmunoassay. Mean concentrations of serum prolactin were 5.57, 10.28, and 10.28 ng/ml at onset (Day 5), middle (Day 14), and end (Day 28) of OC ingestion, respectively. During OC ingestion, prolactin concentration increment was significant (P .001). Raised levels of prolactin persisted until the end of the cycle. In the control group, mean prolactin levels were 5.68, 6.58, 8.79, 7.57, and 6.13 ng/ml on Days 1, 7, 14, 21, and 28 of the menstrual cycle, respectively. Serum estradiol levels were 80.48, 57.17, and 43.05 pg/ml on Days 5, 14, and 28 of oral contraception. Progesterone values were .29, .33, and .45, respectively. When estradiol and progesterone values of normal menstrual cycles (controls) were compared with the OC cycle, the decrease in estradiol was significantly lower and remained so without showing the cyclic pattern of normal menstruation. Serum progesterone levels were similarly affected.