The behavioral effects of subchronic exposure of male Wistar rats to the organophosphorus compound parathion-methyl (for 6 weeks 1/50 or 1/100 of LD50, PO) were studied. Open-field (OF) and elevated plus-maze (EPM) tasks were used to decide whether or not the compound can affect behavior. Significant effects were measured in the OF activity during the first minute, on the activity of crossing outer squares, increasing latencies to leave center, start of rearing, grooming, and defecation. EPM parameters showed an increased amount of time spent in the open arms and a clear tendency to enter more frequently open arms. The defecation rate in the EPM was significantly decreased. Results are discussed in terms of effects on emotional behavioral components (arousal) and on physiological functioning of the peripheral nervous system (lengthening of latencies).