Organophosphorus insecticides (OPI) are the commonest cause of poisoning in Sri Lanka. Being inhibitors of esterases, OPI cause an acute cholinergic crisis as the initial phase of neurotoxicity. The intermediate syndrome (IMS) which develops 24 to 96 hours after poisoning, and the delayed polyneuropathy (OPIDP) which manifests several weeks later, suggest a triphasic effect of OP intoxication. OPI may also cause extrapyramidal manifestations. Chronic neurobehavioural effects of longterm exposure to OPI are also a cause for concern. This paper reviews OPI poisoning in man emphasising the clinical, biochemical, and electrophysiological studies done at Peradeniya over the past two decades.