The purpose of the study is to find out whether treatment with clomiphene citrate (CC) alters the luteal progesterone (P) production pattern. Five female patients with unovulatory infertility under treatment with 100 mg CC taken on days 2-6 of the cycle, were assigned to the study. Blood samples were drawn at 20 minute intervals from 8.00 to 17.00 hrs, seven days after ovulation to assess P and luteinizing hormone levels. P concentrations showed wide fluctuation during the sampling period. Mean P concentration was significantly higher in the morning than in the afternoon (p<0.05). As a conclusion, the diurnal pattern of P production is not disturbed by treatment with CC.