Individual variation in the half-life of caffeine in the body was measured by HPLC analysis of saliva samples. The mean for adult males and non-pregnant females was 3.4 h (range 2-5 h, n = 25), and 8.3 h (range 3-16 h, n = 57) for pregnant women. After delivery, in most cases the values returned to normal within one month. The individual values could not be correlated with age, weight or consumption of coffee. Women drinking large quantities of coffee should be aware of the side effects of coffee during pregnancy, as they may occur at a lower rate of consumption than in the non-pregnant state.