People take up nicotine in various ways, mainly by smoking cigarettes, cigars or pipe, but also by snuffing or chewing tobacco. There is evidence, that the individual uptake of nicotine does not necessarily correlate with the actual content of nicotine in tobacco. Data about nicotine and cotinine concentration in blood and urinary nicotine and cotinine excretion of cigarette-, cigar-, and pipe-smokers will be presented and discussed. It will be demonstrated that measuring cotinine levels in serum may not be an appropriate method for determining the nicotine uptake by smokers.