In 90 male and female patients suffering from recurrent affective disorders who had been stabilised for long periods on lithium carbonate an investigation was carried out to determine factors influencing the dosage and excretion of lithium. The findings support the view that lithium excretion is largely controlled by osmotic factors and that it is a function of the overall renal efficiency. The dosage requirement is a function of both body surface area and weight which are closely correctly with measures of renal efficiency though there are also age and sex differences which are independent of body size.