Renal function, measure as glomerular filtration rate (GFR), sodium excretion and osmolality after thirst, has been determined in untreated (n=35) and treated (n=22) hypertensives and in a reference group (n=80), all derived from a random population sample of 50-year-old men. Renal function was related to casual and resting BP and to relative body weight. Hypertension was defined as SBP greater than 175 or DBP greater than 115 mmHg on two separate occasions or current antihypertensive treatment. Mean GFR was 100 +/- 11.7 ml/min in the reference group and significantly lower, 94 +/- 15.7 ml/min, in the hypertension group. In the hypertension group, 20% had a reduced GFR, although the standard diagnostic procedure, serum creatinine, demonstrated only 4%. Hypertensives with reduced GFR were characterized by higher BP, lower urinary sodium excretion, reversed diurnal rhythm of salt and water excretion and a higher relative body weight, which was, however, explained by the correlation of BP to relative body weight. GFR was negatively correlated to DBP at rest and positively correlated to urinary sodium excretion. Untreated hypertensives with persistent high BP after rest and lower GFR, lower urinary sodium excretion and reversed diurnal rhythm of salt and water excretion, indicating high renal resistance. The results suggest that subjects with relatively severe hypertension as judged by BP and renal function have an increased renal vascular resistance.