The effects of Spirulina platensis on ethanol metabolism were studied in rats. Male Wistar rats aged 5 weeks were fed on a basal diet or a Spirulina -containing diet for 28 days. The blood ethanol level at 30min after intraperitoneal injection of ethanol (200mg/100g body weight) was decreased by Spirulina feeding. To determine whether Spirulina affects ethanol metabolism, alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) activities were measured in rats fed ethanol plus Spirulina for 21 days. Spirulina feeding decreased the Km value of ALDH to acetaldehyde from 0.91mM to 0.70mM. On the other hand, Spirulina feeding did not alter ADH activity. These results suggest that Spirulina may accelerate ethanol metabolism, resulting in rapid clearance of ethanol from blood into the liver.