Acute exposure to the insecticide/acaricide chlordimeform (CDM) produces large, selective and transient changes in visual evoked potentials of rats. Experiments were conducted investigating the influence of physical characteristics of the evoking stimuli on the CDM effect. Adult male Long-Evans rats with epidural visual cortex electrodes were used. In Experiment 1, several alternating square wave grating pattern sizes were used to elicit pattern reversal evoked potentials (PREPs). In Experiment 2, PREPs were elicited with gratings oriented at 0 (horizontal), 45, 90, or 135 degrees. Treatment with 40 mg/kg CDM increased PREP amplitudes and latencies regardless of orientation. In Experiment 3, flash evoked potentials (FEPs), elicited with strobe flashes spanning four log units of intensity, showed a small but significant CDM does X intensity interaction on P2N2 peak-to-peak amplitude.