Twenty-two smoking mothers and their healthy newborn infants (mean postnatal age of 3.7 days) were studied in the maternity ward. A close correlation was found (r = 0.94) between nicotine concentrations in the mothers' plasma and milk after smoking, the milk: plasma ratio being 2.9. The amount of nicotine transferred to the infant increased from 0.09 to 1.03 micrograms/kg infant body weight when mothers smoked before breast-feeding. The daily dose of nicotine via the mothers' milk was 6 micrograms per kg infant body weight. Cotinine but not nicotine concentrations in the plasma and milk of the mothers and the urine of the infants reflected the smoking habits of the mothers during pregnancy. There was no correlation between nicotine and cotinine concentrations in the infant's urine and the amount of nicotine given to the infant via the mother's milk.