Concern for toxicity following exposure to organophosphorus insecticides led us to investigate whether topical application of either malathion or malathion mixed in a protein bait as used for aerial spray applications could be toxic to the ocular/visual system. Adult male Long-Evans rats were either untreated or treated with malathion alone (two drops per day in each eye), bait alone (six drops per day in each eye) or malathion and bait (six drops per day in each eye). The dose levels of malathion alone and malathion and bait were chosen based on pilot work and provided approximately equivalent amounts of active ingredient. The rats were treated 5 days a week for 4 weeks. During the final week of treatment, the rats were implanted surgically with cranial recording electrodes overlying the visual projection area of the cerebral cortex. Visual pattern-evoked potentials (PEPs) were elicited with vertical sinusoidal gratings at three levels of stimulus spatial frequency (0.08, 0.16 and 0.32 cycles per degree) and three levels of visual contrast (0.15, 0.30 and 0.60). After spectral analysis of the PEP waveforms, the amplitude and phase at the stimulus rate (F1) and the first harmonic (F2) were determined. Although F1 and F2 parameters were influenced significantly by manipulation of the stimulus parameters, no significant differences were observed that could be attributed to treatment with the test substances. In addition, an ophthalmological examination of the eyes and a light microscopic evaluation of ocular tissues, including retina and optic nerve, revealed no treatment-related lesions. The dose levels used in this study were high-approximately 84 000 times the exposure per unit surface area expected from aerial spraying-and yet the visual function of the treated subjects was apparently normal. This study identified no significant toxicological concerns regarding direct ocular contact exposure to malathion.