In a preliminary experiment, exposure of rats and guinea pigs to 50 or 11 ppm (v/v) 1,3-dichloropropene vapor for seven hours/day, five days/week for one month had been found to produce liver and kidney injury. In order to determine the effects of more prolonged, repeated exposure to lower concentrations, four species of laboratory animals were exposed repeatedly to nominal concentrations of either 3 or 1 ppm (13.6 or 4.5 mg/m³) 1,3-dichloropropene in air. Rats, guinea pigs, rabbits and dogs received seven-hour daily exposure five days/week to 1 ppm (v/v) (0.9 ppm recovered analytically) for six months with no adverse effect. The only effect in any group exposed to 3 ppm (v/v) (2.6 ppm recovered analytically) was a slight, apparently reversible change seen microscopically in the kidneys of male rats exposed seven or four hours daily. Female rats, male and female rabbits, male and female guinea pigs and female dogs exposed repeatedly to 3 ppm seven hours/day showed no adverse effect. No effects were apparent in either male or female rats killed three months after their last seven-hour exposures to 3 ppm. Based on these data, it is suggested that when repeated, prolonged exposures to the vapors of 1,3-dichloropropene can occur, seven to eight hours daily time-weighted average exposures of workmen not exceed 1 ppm.