The purpose of this investigation was to determine the relationship between core temperature (Tre), tissue insulation (I), and beta-endorphins (beta-END) during immersion in cold water. To test this, 21 males were immersed to the first thoracic vertebra for 120 min in stirred water at either 18 degrees, 22 degrees, or 26 degrees C. Subjects were divided with respect to body fat [high fat (HF) = 18-22%, n = 11; low fat (LF) = 8-12%, n = 10] and randomly assigned to one of the three temperatures. Blood was drawn at 25 and 5 min (T-25 and T-5, respectively) before immersion, after 1 h of immersion, and 5 min before the completion of the second hour (T + 60 and T + 120, respectively). No significant relationship was found between delta beta-END and delta I or delta Tre. The relationship between the changes in thermoregulatory variables and delta beta-END did not differ between the HF and LF groups. Also, beta-END (adjusted for plasma volume) was lower (P < 0.05) in the HF vs. the LF group. beta-END (pg.ml-1) increased (P < 0.05) as time increased from T-25 to T + 60 and were not progressive with duration of immersion. Thus, beta-END did not increase in proportion to level of heat loss.