The aim of this study was to investigate the relevance of individual characteristics for thermoregulation during prolonged cycling in the heat. For this purpose, 28 subjects cycled for 60 min at 60% VO(2peak) in a hot-dry environment (36 ± 1°C; 25 ± 2% relative humidity, airflow 2.5 m/s). Subjects had a wide range of body mass (99-43 kg), body surface area (2.2-1.4 m(2)), body fatness (28-5%) and aerobic fitness level (VO(2peak) = 5.0-2.1 L/min). At rest and during exercise, rectal and mean skin temperatures were measured to calculate the increase in body temperature (ΔT (body)) during the trial. Net metabolic heat production (M (NET)) and potential heat loss (by means of evaporation, radiation and convection) were calculated. Although subjects exercised at the same relative intensity, ΔT (body) presented high between-subjects variability (range from 0.44 to 1.65°C). ΔT (body) correlated negatively with body mass (r = -0.49; P < 0.01), body surface area (r = -0.47; P < 0.01) and T(body) at rest (r = -0.37; P < 0.05), but it did not significantly correlate with body fatness (r = 0.12; P > 0.05). ΔT (body) positively correlated with the body surface area/mass ratio (r = 0.46; P < 0.01) and the difference between M (NET) and potential heat loss (r = 0.56; P < 0.01). In conclusion, a large body size (mass and body surface area) is beneficial to reduce ΔT (body) during cycling exercise in the heat. However, subjects with higher absolute heat production (more aerobically fit) accumulate more heat because heat production may exceed potential heat loss (uncompensability).