More than 1000 Holtzman female rats, weighing 138-144 g each, were used to examine the mechanism(s) of radioprotective action of dimethyl sulfoxide (DMSO). One hundred and ten of these rats were injected intraperitoneally with 2.5-25 g undiluted DMSO/kg body weight. The maximum sublethal concentration of DMSO was found to be approximately 10 g/kg while the LD50 ranged between 15 and 16 g/kg. Five hundred and eight rats were administered x-ray exposures ranging between 250 and 1200 R following an injection with 0, 5, 7.5, or 10 g DMSO/kg. Radiosensitization occurred in some animals administered 10 g/kg, others experienced some radioprotection. Animals injected with 5 or 7.5 g/kg had dose reduction factors of 1.27 and 1.31, respectively. Animals administered 7.5 g/kg appeared to have received the optimal amount of DMSO. The degree of protection or sensitization was found to be dependent upon the DMSO concentration and the radiation exposures, concentration being less important with higher exposures. Three h...