In a study of caffeine and its metabolites in pregnant and non-pregnant women, on average 56.8% of the administered caffeine dose was recovered in the urine in both groups. However, compared to the controls, the pregnant subjects produced smaller amounts of 1-methylxanthine and 1-methyluric acid, whereas the excretion of most of the other metabolites, particularly 3,7-dimethylxanthine and 3-methylxanthine tended to be greater. It is suggested that hormonal influences on the hepatic caffeine metabolising enzymes might be implicated.