Cardiovascular-sympathetic adjustments to nonexertional heat stress in mature and senescent Fischer 344 rats. The purpose of this study was to test the hypothesis that the cardiovascular-sympathetic nervous system adjustments during nonexertional heat stress are exaggerated in senescent (S, 24 mo) vs. mature (M, 12 mo) conscious unrestrained Fischer 344 rats. During two separate trials (48 h apart), each animal was exposed to an ambient temperature (Ta) of 42 degrees C until a colonic temperature (Tco) of 41 degrees C was attained and then cooled at a Ta of 26 degrees C until Tco returned to the initial control level. Trial 1: heart rate (HR), mean arterial blood pressure (MAP), and arterial plasma concentrations of norepinephrine (NE), epinephrine (E), and lactate (La) were similar between the S and M groups during the baseline (control) period. The absolute increases in HR, MAP, NE, and E from the control period to the end of heating were of similar magnitudes between groups; however, La increased more in the S than M animals (P less than 0.05). During recovery, the declines toward control levels for all variables were similar or even more rapid in the S vs. M animals (P less than 0.05). Trial 2: the changes in HR and MAP during heating were similar to those observed in trial 1 in both groups. Generally, NE and E control levels were elevated in both groups compared with those in trial 1. The absolute increases in NE during heating were similar to trial 1 in both groups, whereas E increased to a greater extent than in trial 1 in the S animals (P less than 0.05). 