Output (Qsweat) and lactate concentration ( [L]sweat) in frontal sweat and blood lactate concentration ( [L]blood) were monitored in three male volunteers: one sedentary (S), one fit (F), and one very fit (VF). Measurements were made under three sweating conditions: 1) heat exposure to infrared radiation (IR) at rest for 25 min at 45 degrees C; 2) bicycle exercise at constant relative maximal aerobic power (87 +/- 2% MAP) for 25 min (Cst EX) at 23 degrees C; and 3) bicycle exercise at increasing work loads (from 60-95% MAP) in four or five successive 5-min steps (Incr EX) at 23 degrees C. Comparison of these data showed that under the above three conditions 1) Qsweat of S was lower than that of F and VF; and 2) changes in [L]sweat were different: in IR and Cst EX, [L]blood remained constant, and [L]sweat decreased with heat load duration similarly in the three subjects (the slopes of the two curves were not significantly different); at Incr Ex, [L]blood increased and [L]sweat remained constant in S, F, and VF (21.71 +/- 0.85, 16.75 +/- 0.99, 12.83 +/- 0.45 (SE) mM, respectively). It was suggested that the high [L]blood and increased adrenergic stimulation observed in Incr Ex could act on [L]sweat in relation to the physical fitness level.