This study was designed to examine the anticarcinogenic effect of dietary supplementation with garlic powder on rat hepatocarcinogenesis. All rats were initiated by a single dose (200 mg/body weight) intraperitoneal injection of diethylnitrosamine (DEN), and three weeks later, subjected to two-thirds partial hepatectomy. Two weeks after initiation, four groups of rats were given experimental diets supplemented with 0 (control group), 0.5, 2.0, or 5.0% garlic powder for 6 weeks. Rats were sacrificed at eight weeks after initiation. The induction of placental glutathione S-transferase (GST-P) positive foci was significantly inhibited almost equally in all three groups fed garlic diets. Glucose 6-phosphatase (G6Pase) activity was increased in rats fed 0.5% and 2.0% garlic powder, and was negatively correlated with the number and area of GST-P positive foci. Thiobarbituric acid reactive substance (TBARS) contents were decreased in rats fed 2.0% and 5.0% garlic powder. Only 5.0% garlic powder supplementation significantly increased the glutathione content and the glutathione S-transferase activity, compared to the control group. Therefore, all levels of garlic powder, 0.5% to 5.0%, exerted an anti promotional effect during hepatocarcinogenesis. Dietary supplementation with garlic powder seemed to maintain microsomal membrane integrity by increasing G6Pase activities. Glutathione-dependent detoxifying enzymes did not seem to contribute to this protective effect directly. The present study suggests that garlic powder is effective in inhibiting the induction of GST-P positive foci, possibly by stabilizing the hepatic microsomal membrane.