In this study, the pharmacokinetic behavior of 4-nonylphenol (NP) was investigated in human volunteers. In order to avoid analytical background problems, isotope labeled (13)C(6)-NP was synthesized. Both after intravenous and oral application, the elimination half-life of the parent compound from the blood was 2-3 h. Bioavailability after oral application (determined by oral and intravenous AUCs) was about 20%. NP seems to distribute into the lipid phase of the body within 2 h. Furthermore, levels of NP and 4-octylphenol (OP) in non-occupationally exposed persons were investigated by analyzing human autopsy adipose tissue samples. NP concentrations ranged from 19 to 85 ng/g lipids, OP concentrations from 0.58 to 4.07 ng/g lipids. These values were both in the range of the analytical background contamination. No NP and OP ethoxylates (ethoxylate number 1,2) were found in any of the samples (detection limit of 10 ng/g lipids for NP ethoxylates and 0.5 ng/g lipids for OP ethoxylates). On the basis of the pharmacokinetic data from this study, actual adipose tissue concentrations were estimated to lie a factor of 50 below analytical background values.