The exposure of infants to nicotine via milk of smoking mothers or via inhaled side-stream smoke ("passive smoking") was evaluated. Newborn infants nursed by smoking mothers and unexposed to passive smoking showed measurable serum concentrations of nicotine (0.2 to 1.6 ng/ml) and its main metabolite, cotinine (5 to 30 ng/ml), and also excreted measurable amounts of nicotine and cotinine in their urine: the ratio of nanograms of nicotine/milligrams of creatinine (N/C ratio) ranged from 5.0 to 110 (median 14), and the corresponding ratio of nanograms of cotinine/milligrams of creatinine (C/C ratio) from 10 to 555 (median 110). Infants of the same age nursed by nonsmoking mothers did not excrete measurable amounts of the two substances except in one case. Older and non-breast-fed infants exposed only to passive smoking had N/C ratios in the range of 4.7 to 218 (median 35) and C/C ratios in the range of 117 to 780 (median 327 ng/mg). Infants exposed to passive smoking and to smoke via breast milk had N/C ratios in the range of 3.0 to 42 (median 12) and C/C ratios in the range of 225 to 870 (median 550). The significant serum concentrations and urinary excretion rates of nicotine in the breast-fed infants of smoking mothers suggest that nursing contributes to the nicotine exposure of these neonates. In older infants, the wide variation of cotinine excretion values did not allow separate evaluation of the two exposure routes.