Deliveries of tar, nicotine and carbon monoxide in mainstream smoke decreased as the exogenous-potassium content of tobacco increased. The puff count was almost unchanged with added potassium malate, but slightly decreased with added nitrate. Potassium malate was more effective than the nitrate for the reduction of carbon monoxide, whereas the opposite was observed in the case of nicotine. Both the exogenous and the endogenous potassium contents influenced the reduction of carbon-monoxide delivery through a decrease in the peak temperature of the burning cone. The pungency and off-taste peculiar to the tobacco were eliminated to some extent by the addition of potassium malate at levels less than 2 % expressed as potassium.