The effect of oral cimetidine on plasma lipid and lipoprotein levels was evaluated in a double-blind crossover study in 12 normal subjects and in 12 subjects with fasting serum glucose levels greater than or equal to 140 mg% or levels after a 100-gm oral glucose dose greater than or equal to 170 mg%. Plasma cholesterol (total, high-density, low-density, and very low-density lipoprotein) and triglyceride (total and very low-density lipoprotein) concentrations in placebo and cimetidine periods did not differ. Cimetidine was associated with elevated serum creatinine and aspartate amino transferase (SGOT) levels. One subject was impotent while on cimetidine.