Centchroman, a non-steroidal oral contraceptive drug, was given to 13 nursing mothers comprising two groups. Each participant in group I (n = 8) received a single 30 mg dose, and in group II (n = 5) each participant received a 30 mg twice a week dose for twelve weeks. Simultaneous blood and milk samples were collected and analyzed for the parent drug by high performance liquid chromatography. In the single dose study (group I), the mean +/- peak centchroman concentrations in milk and serum were 78.7 +/- 28.4 and 63.6 +/- 23.6 ng/ml with milk-to-serum (M/S) ratio of 1.4 +/- 0.9. There was no significant increase in centchroman concentrations in milk after multiple dosing (group II). However, serum concentrations reached up to 112.5 ng/ml at 6 h after the 13th dose. Average M/S ratios were insignificantly different at trough (prior to next dose) and at peak (4-6 h after dose) centchroman levels. Additionally, the breast milk and serum centchroman concentrations showed a significant correlation (r = 0.64, P < 0.01), indicating that the amount of centchroman excreted into breast milk is dependent on serum concentrations. The weekly dose (% of the maternal dose) of centchroman ingested by the breast-fed infant at peak maternal serum and milk levels was in the range of 0.4 to 11.5%, assuming a weekly milk uptake of 1.05 l/kg. There was no significant difference in the dose ingested by the infants between the two dosing groups. These levels of centchroman passing into breast milk and subsequent exposure to the infants are unlikely to be of any physiological consequence.