The present experiments demonstrated a reliable within-session increase in rectal temperature (T(re)) at 25 degrees C during stable differential-reinforcement-of-low-rate (DRL) performance. The thermal response was found to be independent of the DRL value and reinforced DRL performance, but dependent on the state of the animal's deprivation. Exposure to a 35 degrees -C environment increased the post-session T(re) significantly above the 25 degrees -C post-session T(re) for seven of eight subjects. Response and reinforcement rate at 35 degrees C was found to be independent of DRL value, although it decreased as DRL value increased at 25 degrees C. A discriminative stimulus used to mark the end of the interval increased the reinforcement rate at 25 degrees C but provided no advantage at 35 degrees C. Measurement of the pattern of responding during DRL performance revealed increases in the proportion of long interresponse times at 35 degrees C. Reinforcement rate was found to decrease progressively at 35 degrees C, reaching a minimum within 40 to 50 min of the 90-min session. Visual observation of overt behaviors during DRL performance at 35 degrees C revealed a reduction in the frequency of overt behavior, characteristic of 25 degrees -C performance, and a time-dependent increase in the probability of an alternative set of behaviors.