Thermoregulatory responses of eight healthy males (age 25.5 ± 4.5 yrs) were studied during weight training comprising 3 sets of 15 repetitions of 9 exercises performed at a work cadence of 15 repetitions.min-1 with 1-min recovery intervals. The load for each exercise was increased from 50% of the 15-repetition maximum for the first set to 75% and 100% for the second and third sets, respectively. The thermoregulatory response was characterized by only moderate sweat rates (0.69 ± 0.18 l.h-1) and rectal temperature rises (1.3° ± 0.4°C, P < 0.001), suggesting that dehydration and hyperthermia are unlikely to complicate weight training of the format used in this study. Despite a considerable lactic acidosis, small elevations in serum levels of aspartate aminotransferase and lactate dehydrogenase occurred, the core temperature rise being inadequate for significant cellular damage to ensue. Serum electrolyte levels measured immediately and 24 h post-exercise indicated that electrolyte supplementation is unlikely to be of benefit. Weight training induced a marked reduction of plasma volume (11.8% ± 3.7%, P < 0.001) in the presence of a minor water deficit (0.8% ± 0.23%) and an O2 consumption of 32% ± 8% of the predetermined treadmill exercise maximal O2 consumption. This finding suggests that exercise intensity as assessed by percentage maximal voluntary contraction rather than percentage maximal O2 consumption might determine the degree of hemoconcentration encountered during exercise.