The serum concentrations of 3-ketodesogestrel (KDG) and gestodene have been measured in 30 and 31 women respectively who took low dose oral contraceptives containing 30 micrograms ethinylestradiol together with either 150 micrograms desogestrel or 75 micrograms gestodene for 6 months. On days 1, 10 and 21 of the first third and sixth treatment cycles blood samples were drawn at 0, 0.5, 1, 1.5, 2, 3, 4 and 24 h. KDG and gestodene levels were measured by radioimmunoassays and were evaluated for Cmax (peak serum concentration), tmax (time to Cmax), and AUC (area under the curve) to 4 and 24 h. The overall total gestodene concentrations were higher and the accumulation of the steroid throughout a cycle greater than that of KDG. For example, the AUC0-4 of gestodene increased in cycle 1 by a factor of 2.8 (day 10 vs. day 1) and 3.6 (day 21 vs. day 1) compared to 2.3 and 2.6 for KDG. The higher concentration of gestodene reflects a lower volume of distribution than KDG, and is consistent with gestodene binding to sex hormone binding globulin (SHBG) with a higher affinity than KDG. Concentrations of KDG and gestodene were higher on day 1 of cycles 3 and 6 than on day 1 of cycle 1. The serum concentrations of KDG and gestodene during multiple dosing cannot be predicted on the basis of single dose pharmacokinetics.