To study the cognitive changes following chronic occupational exposure to organophosphate (OP) pesticides, a clinical and neurophysiological study was performed on 31 workers engaged in spraying fenthion, an OP pesticide. The mean age of the workers was 32.1 years (range 19-55) and mean duration of exposure 10.5 years (range 1-14). The workers reported mild transient symptoms after spraying. There was no clinical evidence suggestive of excessive cholinergic activity. Clinical psychometry revealed significant changes in Benton visual retention test, memory quotient, and Alexander's Passalong Test. Serum AChE level was 27% less in the exposed group compared to the controls. P3 of event related potentials were elicited in 28 subjects, P3 latency though was prolonged in one subject only but the group difference was significant. The amplitude of P3 however did not show significant difference. The results suggest subtle subclinical effect of chronic fenthion exposure on the cognitive functions and event related potentials.