Cotinine was measured in the amniotic fluid and urine of 31 pregnant women and in the urine of their offspring. Amniotic fluid cotinine was 8 times higher in active and 2.5 times higher in passive smokers than in non-smokers. In general, amniotic fluid cotinine was considerably higher than urinary cotinine both in active and in passive smokers. Estimation of cotinine both in amniotic fluid immediately before delivery and in urine of the newborn on the 1st day of life aids in assessing the degree of prenatal exposure to tobacco smoke.