Cynomolgus macaque monkeys received head-only exposure to 0, 100, 200, 500, 1000, 2000, 3000, and 4500 ppm toluene for 50 min while simultaneously tested for delayed matching-to-sample behavior, a test of cognitive functions. Response time increased and accuracy of matching decreased at 2000 ppm or more of toluene, indicating an attentional deficit but not specific memory effects. Behavioral indices exhibited monotonic concentration-related changes. Expired carbon dioxide (CO2), the most sensitive index, displayed an inverted U-shaped concentration-effect curve, which increased at 100 ppm (the TLV) and decreased at 4500 ppm toluene. Changes in expired CO2 provide new evidence of physiological changes at very low levels of toluene. These changes may indicate combined behavioral, respiratory, sensory, and metabolic effects. No behavioral measure exhibited either cumulative effects or tolerance to 4500 ppm during two 3-day exposures. However, both response time and expired CO2 exhibited an acute, within-session tolerance. The results indicate that brief inhalation exposure to toluene impairs cognitive and motor abilities at concentrations below those causing overt signs, such as ataxia and intention tremoring.