Interleukin-3 (IL-3) is one of a number of colony stimulating factors which are known to regulate hematopoiesis and cell death (apoptosis). In the present study, however, we examined another possible function of IL-3, namely, whether it affects diaphragm contractile properties in rats. In the saline+endotoxin group, the force-frequency curves decreased significantly at 4 hours (1.63±0.06 kg/cm², p<0.05) from those at 0 hours (1.84±0.07 kg/cm²) ; muscle fibers which showed increased NADPH diaphorase staining were types I (SO) and IIa (FOG). In the IL-3+endotoxin group, the force-frequency curves did not show significant changes between 0 hours (1.72±0.15 kg/cm²) and 4 hours (1.70±0.05 kg/cm²). In the IL-3 only group, the force-frequency curves at 4 hours (1.78±0.10 kg/cm², p<0.05) decreased significantly from those at 0 hours (2.12±0.06 kg/cm²) ; muscle fibers which showed increased NADPH diaphorase staining were also types I (SO) and type IIa (FOG). IL-3 itself significantly decreased force-frequency curves and induced NO production in the diaphragm muscle fibers at 4 hours. From these results, IL-3 prevented the deterioration in force-frequency curves and NO production induced by endotoxin administration, therefore, it is suggested that IL-3 has a cell protecting effect coexisted with endotoxin.