The aim of this study was to determine the most appropriate urine collection for detecting differences in the excretion rates of albumin, gamma glutamyl transpeptidase (GGT) and N-acetyl-beta-D-glucosaminidase (NAGA) between normotensive subjects and hypertensive patients on treatment. Twenty treated hypertensive patients, mean (SEM, standard error of mean) age; 52.2 (6.2) years and 20 normotensive subjects, mean age 49.2 (4.2) years, were studied in a consecutive sampling design. Urinary excretion rates of albumin, GGT and NAGA were determined in consecutive timed urine samples collected overnight and during 3-5 h the next morning. Mean (SEM) overnight excretion rates for albumin, GGT and NAGA for normotensive subjects were 11.05 (1.18) micrograms/min, 17.00 (2.20) mU/min and 6.55 (0.39) mU/min, respectively, which were significantly lower than those of hypertensive subjects which were 20.77 (2.14) micrograms/min, 21.84 (1.65) mU/min and 10.92 (0.87) mU/min, respectively (P < 0.05). The mean (SEM) percentage increases in urinary albumin, GGT and NAGA in morning urine collections of normotensive subjects of 15.22 (3.88)%, 34.04 (6.45)% and 11.54 (3.63)%, respectively were significantly lower than 107.03 (15.04)%, 121.96 (16.71)% and 72.75 (7.50)% found in hypertensive patients (P < 0.05). These data suggest that were urinary albumin and tubular enzyme excretion to be used as correlates of hypertensive renal damage, ambulatory urine collections may be more sensitive than overnight collections.