Abstract Dicyclopentadiene [3a,4,7,7a-Tetrahydro-4,7-methanoindene] is slightly to moderately toxic by the dermal route and highly toxic by the oral route in single-dose studies. The LC50 values for single 4-hr vapor inhalation periods varied from 771 ppm for male rabbits to 145 ppm for male mice. Human sensory response studies revealed that the vapor has very good warning properties as it was detected at 0.003 ppm. The response of rats and dogs to repeated 7-hr exposures of dicyclopentadiene vapor for 89 days suggests that man should not be allowed to inhale more than 5 ppm during an 8-hr working day, 5 days per wk. It should be emphasized that this “hygienic standard” is an estimate based on experimental data and therefore is subject to modification as information is gathered on human experience.