Analysis of 44 milk samples from 23 nursing smokers revealed that there was a linear correlation between nicotine concentrations in serum and in milk (r = 0.70). The nicotine concentrations in milk were considerably higher than the corresponding serum concentrations: milk/serum concentration ratio = 2.92 +/- 1.09; (n = 44). There was also a linear correlation between the cotinine concentrations in serum and in milk (r = 0.89). The cotinine concentrations in milk were lower than the corresponding serum concentrations: milk/serum concentration ratio = 0.78 +/- 0.19; (n = 44). The direct comparison between the half-lives of nicotine in serum and in milk was possible in five nursing smokers. The half-life of nicotine in milk was determined in four additional smoking mothers. The half-life of nicotine in milk t 1/2 = 97 +/- 20 min slightly exceeded the half-life of nicotine in serum t 1/2 = 81 +/- 9 min; the difference between these two values was not statistically significant (P greater than 0.05). Cotinine concentrations remained fairly consistent during a 4 h interval without smoking.