The influence of working or swimming in indoor swimming pools on the concentrations of four trihalomethanes (haloforms) in blood and urine was investigated. Different groups (bath attendants, agonistic swimmers, normal swimmers, sampling person) were compared. The proportions of trihalomethanes in blood and urine correlated roughly with those in water and ambient air. Higher levels of physical activity were correlated with higher concentrations. Within one night after exposure in the pool the blood concentrations usually were reduced to the pre-exposure values. Secretion of trichloromethane in urine was found to be less than 10%.