Stress-induced change in the distribution of the drug pyridostigmine (PYR) has been proposed as a contributing factor to unexplained illnesses in Persian Gulf War veterans. We evaluated the effects of two stress models, forced running and forced swimming, on acute PYR (30 mg/kg, p.o.) toxicity and cholinesterase (ChE) inhibition in the blood and selected brain regions of young adult male Sprague-Dawley rats (6 weeks of age). Plasma corticosterone levels were measured at 0, 1 and 3 h after termination of forced swimming or forced running to confirm the induction of stress. PYR was given either immediately before stress (15 min swimming; 20 min running) or immediately after stress (15 min swimming; 90 min running) and cholinergic toxicity and ChE inhibition were evaluated at 1, 2 or 4 h after PYR exposure. Additionally, rats were subjected to either swimming (15 min) or running (90 min) stress, anesthetized, injected with horseradish peroxidase (HRP, 100 mg/kg, transcardial) and brain-regional HRP activity measured as an indicator of altered blood-brain barrier integrity. Both forced swimming and forced running resulted in significant elevations of plasma corticosterone levels. PYR caused cholinergic toxicity at all time-points evaluated. Swimming and running stress had little influence on expression of PYR-induced toxicity, however. Blood ChE activity was generally inhibited 77-91% at 1-4 h after PYR, but rats pretreated with PYR prior to forced swimming showed lesser inhibition (64%) 1 h after dosing, possibly because of swimming-induced hypothermia and delayed absorption of the drug. Minimal changes in ChE activity were noted in frontal cortex, cerebellum and hippocampus following PYR exposure (maximal inhibition 28%), and neither swimming nor running stress affected the degree of inhibition. Neither stress model increased HRP accumulation in any brain region. The results suggest that stress associated with forced running or forced swimming has little effect on acute PYR toxicity, entry of PYR into the brain or PYR-induced brain-regional ChE inhibition.