The effects of the discontinuation (DET) of an endurance training/heat acclimation (T/A) program on vascular volumes were studied in 16 adult males. Resting and exercise blood volume dynamics were examined prior to and during an exercise task performed after completion of T/A (CT1) and again at the end of DET (CT2). T/A consisted of cycling at 60% of peak VO2 for 90 min per day, 6 days per week, for 4 weeks. Ambient temperature was 20 degrees C for the first 3 weeks and 40 degrees C for the last week (rh = 30-35%). Subjects were randomly assigned to one of the following DET conditions: 1) cycling one day per week at 40 degrees C, 2) cycling one day per week at 20 degrees C, 3) resting one day per week at 40 degrees C, 4) control. The exercise tasks consisted of 60 min of continuous cycle ergometer exercise at 50% of peak VO2 (Ta = 30 degrees C, rh = 35%). Although significant differences were found between CT1 and CT2, there were no interactions between the various DET conditions. Resting red cell volume decreased 98 ml and plasma volume decreased 248 ml following DET. A reduction in plasma protein content accounted for 97% of the decrease in plasma volume. Hemoconcentration occurred during exercise in both CT1 and CT2, while there were slight increases in plasma [Na+] and [Cl-] and a rapid rise in [K+]. It appears that a single exercise and/or heat exposure per week was not different from complete cessation of endurance exercise in the heat with regard to maintenance of the various vascular volumes.