Diurnal cardiovascular patterns in spontaneously hypertensive and Wistar-Kyoto rats. This study was designed to determine whether diurnal patterns of blood pressure, heart rate, or locomotor activity differed among two substrains of Wistar-Kyoto rats, derived originally from Charles River or Taconic Farms stock, or the spontaneously hypertensive rat. Cardiovascular parameters were continuously monitored over 24 hours. Resting systolic and diastolic blood pressure values were statistically different among the three groups both during the lights-on (rest) and lights-off (active) phases of the cycle with blood pressure of spontaneously hypertensive rats greater than that of Wistar-Kyoto rats from Taconic Farms, which was greater than that of Wistar-Kyoto rats from Charles River. The largest difference in arterial pressure between Wistar-Kyoto/Taconic Farms and Wistar-Kyoto/Charles River was during the lights-on period. Heart rates of all rats decreased during the lights-on period; Wistar-Kyoto/Charles River had the largest decrease (-70 +/- 5 beats/min), Wistar-Kyoto/Taconic Farms had the least (-17 +/- 2 beats/min), and in spontaneously hypertensive rats the decrease was intermediate (-29 +/- 3 beats/min). The pronounced diurnal variation in pressure and heart rate exhibited by Wistar-Kyoto/Charles River was not present in either Wistar-Kyoto/Taconic Farms or spontaneously hypertensive rats. Blood pressure magnitude correlated with locomotor activity during both periods, although all groups showed minimal activity during the rest period. Observed differences between Wistar-Kyoto/Charles River and Wistar-Kyoto/Taconic Farms were not due to a lack of or an abnormality in baroreceptor reflex function. 