Five subjects exercised on a cycle ergometer for 30 min at 55% peak oxygen consumption on two occasions in an environmental test chamber (ambient temperature = 29 degrees C; dew point temperature = 10 degrees C). Pyridostigmine bromide (PYR), an acetylcholinesterase (AChE) inhibitor, was ingested (30 mg) approximately 150 min before one experiment, and no drug was administered during the other experiment (control). Red blood cell AChE inhibition averaged 40 (+/- 7)% during PYR treatment. Esophageal temperature (Tes), an eight site-derived mean skin temperature, forearm blood flow (FBF; venous occlusion plethysmography), skin blood flow (SkBF; laser-Doppler velocimetry), and metabolic rate (indirect calorimetry) were measured. SkBF decreased 37% after PYR treatment compared with control (P less than or equal to 0.05). The Tes threshold for initiation of cutaneous vasodilation was 36.8 (+/- 0.3) degrees C for the control treatment and 37.0 (+/- 0.3) degrees C for the PYR treatment (P less than or equal to 0.01). FBF was not significantly different between treatments, whereas heart rate was reduced by 7 and 9 beats/min during rest and exercise, respectively (P less than or equal to 0.01). The increased threshold for initiation of cutaneous vasodilation with AChE inhibition by PYR is compatible with nonthermal modulation of the control of thermoregulation through increased acetylcholine (ACh) accumulation. This could potentiate preganglionic transmission to enhance adrenergic vasoconstrictor tone. One suggested mechanism possible at the neuroeffector junction of the sweat gland may be that accumulated ACh diffusion across the adventitia of adjacent arterioles to muscarinic receptors initiates contraction of the smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)