Studies "in vivo" and "in vitro" of methemoglobin formation with acetylphenylhydrazine has been done. Heilmeyer's method has been used in the methemoglobin determination. "In vitro" 0.1 gr. acetylphenylhydrazine in 0.1 ml blood, produces after 15 minutes, 9% methemoglobin. "In vivo", in quantities aproximately 5 times less (30 mg per kilo body weight), no methemoglobin formation has been observed in the first six hours after the drug have been administered.