Renal function tests were performed in 101 unselected patients who had been on lithium for two weeks to 12 years. None had a recorded episode of lithium intoxication. The glomerular filtration rate (GFR), was not correlated with the cumulative dose of lithium or the duration of use of other psychotropic drugs. Nine patients had creatinine clearances lower than predicted; in six of these, no cause was identified and the small reductions in GFR may have been related to lithium use. Urinary concentrating ability (Umax) declined with age, and total dose of lithium received. Although the concurrent use of neuroleptics did not significantly reduce the Umax, the total duration of treatment with these drugs showed a negative correlation. The results suggest that prolonged use of neuroleptics, particularly in patients treated with lithium, may be responsible for an irreversible reduction in urine concentrating ability. Microalbuminuria was present in 40 per cent of the patients, although the rate of albumin excretion was not correlated with duration of use of psychotropic drugs. beta 2 microglobulin excretion was only raised in nine of these patients, suggesting that increased glomerular permeability rather than impaired proximal tubular protein reabsorption was responsible for the proteinuria. The urinary excretion of beta 2 microglobulin and N-acetyl-beta-glucosaminidase were slightly increased in small numbers of patients, indicating little evidence for proximal tubular damage. Increased NAG excretion did not correlate with reduced distal tubular function. However, there was a tendency to higher urinary beta 2 microglobulin excretion in patients with a reduced Umax.