['To study the effects of postnatal exposure to nicotine on the regulation of heart rate and blood pressure in infants.', "Thirty-eight mother-infant pairs were studied. Twenty nonsmoking and 18 smoking (2-20 cigarettes per day) mothers were included. All infants were healthy, exclusively breastfed and their postnatal age was 6 weeks. During a home visit infant's urine and mothers' milk were sampled and concentrations of nicotine and cotinine were analyzed. Infants' electrocardiogram (ECG) were recorded, sleep state documented and blood pressure during sleep was measured. Heart rate variability (HRV) was calculated with spectral analysis of R-R intervals.", "The smoking mothers exposed their infants to nicotine in milk with a median nicotine concentration of 47 (8-192) mug/L. Analysis of infants' urine showed that the nonsmoking group had 0.8 (0-5.2) and the smoke group 60 (17-139) mug cotinine/L (p < 0.01). The frequency domain low-to-high frequency (LF/HF) ratio, was correlated to milk nicotine concentrations in the milk sample, from smoking mothers. HRV decreased, with increasing milk nicotine, ingested by the boys (r =-0.74, p = 0.02) but not the girls (r =-0.13, p = 0.76). The differences of mean arterial pressure between sleep states in the infants, were significantly lower in the smoke group 5.8(6.8) compared to the nonsmoke group 11.5(7.2) mmHg (p = 0.03)."] Does nicotine in breast milk influence heart rate variability in the infant?