Women who smoke and breast-feed pose an unknown threat to their infants' health. In this pilot study, relationships between ingestion of nicotine in breast milk and physiologic effects in the infant were investigated. Infant physiologic effects measured were temperature, pulse, respiration, systolic blood pressure, and oxygen saturation. Five smoking and five nonsmoking mother-infant pairs were studied. Breast milk was analyzed for nicotine using gas chromatography. Breast milk from smoking mothers contained a mean of 33.1 ng/mL of nicotine while the breast milk from nonsmoking mothers contained a mean of less than 6.45 ng/mL of nicotine. Infant physiologic measures were taken before and 20 min after breast-feeding. After breast-feeding, infants of smoking mothers had a significant change in respirations and oxygen saturation while infants of nonsmoking mothers had a significant change in pulse only. Results provide a scientific basis for counseling smoking, breast-feeding mothers.