Systolic (SBP) and diastolic blood pressure (DBP), heart rate (HR) and locomotor activity have been measured at 1-min intervals for 24 h in Sprague- Dawley (N=5) and for 2x24 h in transgenic hypertensive (N=4) rats. The animals were freely mobile and entrained to a 12:12 LD cycle (lights on at 0700). The endogenous circadian component of the cardiovascular variables was removed from the raw data, and then correlations between activity and the residual component (raw data minus the endogenous component) of SBP, DBP, and HR were calculated. This calculations was performed twice, in the mid-light and mid-dark phases. We have investigated if the mean size of the correlation coefficients depended on cardiovascular variable (SBP, DBP or HR), phase (D or L) or strain (Sprague-Dawley, SPD, or Transgenic, TG, rats). Nearly all correlations were positive and ANOVA's showed a significant effect of cardiovascular variable for both strains, with correlations for HR being significantly higher than those for SBP and DBP. Then mean correlations in the SPD strain were significantly higher than in the TG strain for variables SBP and DBP, but not for HR. The correlations between activity and blood pressure were more marked for SPD rats in the light (inactive) than dark (active) phase. Both strains showed ultradian rhythms in all variable, particularly in the light phase. If the analysis was repeated using deviation of the cardiovascular variables from a 1-h moving average rather than endogenous circadian component, then the results were very similar. The results are discussed in terms of the links between the rhythms of activity and cardiovascular variables, with particular reference to differences between the two strains.