Circadian blood pressure (BP), heart rate (HR) and motor activity (MA) of nondiabetic (nd) and spontaneously diabetic (d) BB/OK rats were compared with that of spontaneously hypertensive rats (SHR). In addition a diabetes-resistant and non-hypertensive rat strain (LEW.1W) was monitored for the same parameters. Systolic and diastolic BP (SBP, DBP), HR and MA were measured telemetrically. In d BB rats, the 24 h mean value of SBP (132 +/- 0.15 mm-Hg) was significantly increased compared to nd BB rats (125 +/- 0.18 mmHg). No differences were found in DBP between d and nd BB rats (93 +/- 0.13 v.s. 94 +/- 0.15 mmHg). Both, d and nd BB rats were significantly different in SBP and DBP to that of SHR (155 +/- 0.19 and 110 +/- mmHg). Nondiabetic BB rats did not significantly differ from LEW.1W rats in SBP and DBP (125 vs. 123 mmHg and 94 vs. 94 mmHg). The heart rate was lowest in diabetic BB rats compared with all other strains. Compared to nd BB the diabetic rats had an altered daily rhythm in BP. The results demonstrate that the diabetic BB rats develop circadian variations in BP and HR similar to those observed in hypertensive rats.