Young adult male and female rats and male beagle dogs were exposed to phenylphosphine for six hours daily, five days per week, for a total of 59 exposures. Two test levels and a control were used. The test levels were designed as 0.3 and 3.0 ppa (viv), but averaged 0.6 and 2.2 ppm over the 90-day period. At 0.6 ppm, mild clinical and hem:tologic effects were observed in some anamals; however, histopathologic examination of tissues revealed no test compound-related effects. Rats exposed to the 2.2 ppm level showed severe clinical, hematological, and histopathologic effects. Dogs exposed at this level vere more affected than those at the lower level; however, they were less severely affected than rats exposed to 0.6 ppm. Testicular degeneration observed in rats at the high level was irreversible, while in dogs it showed a slight reversal. Two and 1/S parts per million (2.2 ppm) phenylphosphine is a definite chronic affect level based on data collected during this study, while 0.6 ppm appears to be a threshold level for effect. The possibility of species differences must always be considered and a more susceptible species may be affected at a lower dose level dban the rats were in this experiment. It follow chat all results cannot be extrapolated to humans, a species for which no phenylphosphine exposure data are available.