In a cross-over study, 24 mmol of lithium sulphate was given as a single dose in slow release tablets to 30 healthy volunteers fasting and after a standardised meal. Comparisons were also made with lithium citrate in slow release tablets and placebo. Postprandial administration of lithium gave practically no side effects, while lithium on an empty stomach gave diarrhoea in about 20% of the subjects. The absorption was measured by determination of the amount of lithium excreted in the urine in a group of ten subjects. Lithium was completely absorbed when given after food, but when given on an empty stomach the absorption was lower in some subjects, apparently due to rapid gastrointestinal passage in connection with diarrhoea. Lithium should therefore preferably be administered after meals.