Nondiabetic hypertensive patients lacking the normal nocturnal decline in arterial blood pressure have enhanced cardiovascular complications. Since cardiovascular morbidity and mortality are increased in non-insulin-dependent diabetes mellitus (NIDDM), we performed a prospective cross-sectional case-controlled study comparing the diurnal variation in arterial blood pressure, prevalence of dippers, cardiac autonomic nervous function (beat-to-beat variation during deep breathing), and extracellular fluid volume (51Cr-labeled EDTA) in 55 NIDDM patients with diabetic nephropathy (group 1), 55 NIDDM patients with normoalbuminuria (group 2), and 22 nondiabetic control subjects (group 3). All antihypertensive treatments were withdrawn at least 2 weeks before the study. The nocturnal blood pressure reduction (daytime-to-nighttime)/daytime (mean +/- SE) was impaired in group 1 (6.6 +/- 1.5%) and group 2 (11.1 +/- 1.4%) as compared with group 3 (17.6 +/- 1.7%), and it was impaired in group 1 as compared with group 2 (P < 0.05 for each comparison). The prevalence of dippers (95% confidence interval) was lower in group 1 (42% [29-56]) as compared with group 2 (58% [44-71]; P = 0.08) and group 3 (86% [65-97]; P < 0.001) and in group 2 as compared with group 3 (P < 0.01). Abolished beat-to-beat variation was more prevalent in group 1 (63% [50-76]) as compared with group 2 (15% [7-27]) and with group 3 (5% [0-23]) (P < 0.001). Nocturnal blood pressure reduction was associated with beat-to-beat variation during deep breathing (r = 0.22, P < 0.01). Extracellular fluid volume (mean +/- SE) was higher in group 1 (15.9 +/- 0.5 l/m2) as compared with group 3 (14.1 +/- 0.8 l/m2) (P < 0.05) with group 2 between the two (15.1 +/- 0.4 l/m2).(ABSTRACT TRUNCATED AT 250 WORDS)