A technique is described in which the toxicokinetics of paraquat on passage through the heart and lung were studied by means of blood samples drawn simultaneously from the pulmonary and radial arteries in six cases of acute human life-threatening paraquat intoxications. There was a trend for the radial plasma concentrations of paraquat to be greater than or equal to the pulmonary concentrations. This data suggests that from a clinical point of view the distribution of paraquat into the lungs is not a slow process. There may also exist some efflux of the paraquat into the circulation. This efflux of paraquat may result from direct lung and/or heart injury induced by massive paraquat poisoning.