Blood pressure and urinary protein excretion were monitored in male fawn hooded rats (FH rats) from 8 until 46 weeks of age. Mild hypertension was already observed at 8 weeks of age. Between the age of 5 and 7 months the blood pressure rose steeply to a plateau of about 200 mmHg. Then it stabilized and the level was different for animals of different litters but similar for littermates. Concomitantly with the increase in blood pressure, proteinuria increased with age. Examination of renal tissue at 6.5 and 9 months of age revealed the presence of focal and segmental glomerulosclerosis. The renal changes were not accompanied by gross alterations in renal function. In animals with severe hypertension pronounced proteinuria occurred, and they appeared to form a distinct class. In some of the animals intermittent haematuria occurred. Persistent haematuria, however, had a bad prognosis. There was no glucosuria. Water intake of the animals with severe hypertension was increased. Water intake of young FH rats was found to be of value for predicting the severity of the hypertension in these animals at a later age. It is concluded that the FH rat is an example of a non-inbred rat strain showing spontaneous hypertension. This hypertension may result from an aberrant renal water handling and/or volume regulation.