This study extends previous observations of the conditions under which enhancement of lymphocyte activity occurs following cold swim stress and presents a possible explanation for the enhancement observed. Eight- to twelve-week old male Sprague-Dawley rats swam for 10 minutes daily for one, three, or five days in cold water at 15 degrees C and were killed 0, 30, or 240 minutes following the last swim. Apparatus control animals were placed into an empty swim tank for 10 minutes and then returned to their home cages. Home cage control animals were not manipulated experimentally at all. Splenocyte but not thymocyte responses to concanavalim A were significantly enhanced after one, three, and five days of stress. This enhancement was seen after 0, 30, and 240 minutes of recovery and also in the apparatus controls! The number of splenocytes did not change significantly, but thymocyte number declined following the swims. The blood displayed no changes in leukocyte percents. Serum corticosterone levels were significantly higher and serum testosterone levels were significantly lower after one, three, and five days of stress. The drop in testosterone levels may have released the lymphocytes from inhibition by this hormone, resulting in increased responsiveness. There were significant elevations in levels of blood glucose and protein following one, three, and five days of stress sessions, correlated with the increases in serum corticosterone.