Rubratoxin B, a metabolite of Penicillium rubrum, is a potent mycotoxin that produces hepatic mid-zonal necrosis. Using propylene glycol as the solvent, the acute LD50 (i.p.) in male mice was estimated at 1.42 mg/kg. Pretreatment with s.c. pellets of pentobarbital reduced this toxicity approximately 42 percent, while castration and SKF 525A pretreatment enhanced toxicity approximately 20 percent and 60 percent, respectively. These studies also demonstrated that rubratoxin B potentiated pentobarbital-induced narcosis and enhanced pentobarbital hypothermia. The effect of SKF-525A and pentobarbital on the LD50 value of rubratoxin B suggests that the toxicity of this mycotoxin in mice does not arise from a toxic metabolite but from the parent compound.