Several years ago the authors studied the acute and subacute toxicity of hexamethylphosphoramide (HMPA) (Kimbrough and Gaines 1966). HMPA had been suggested as a chemosterilant for houseflies (Chang et. al. 1964). In the studies atrophy of the testes was observed in adult male rats fed 750 ppm (80-40 mg/kg/day) in the diet for 61-103 days. A high incidence of pneumonia was also found in the poisoned animals. This finding was further investigated (Kimbrough, R.D. and Sedlak, V.A, 1968). Corresponding observations were made by Jackson et al. (1966, 1969). Shott et al. (1971) reported a reproduction study where 2 and 10 mg/kg HMPA were given to male and female rats daily for 169 days. Reproduction was not affected but a high incidence of murine pneumonia was observed. Following a preliminary study (Kimbrough and Gaines 1966) the authors investigated the toxicity of HMPA in a long term study in rats with lower dietary levels. It is the purpose of this communication to report very briefly the main findings of that study.