AbstractAbnormal porphyrin metabolism can be induced by several chemicals. To investigate the synergistic effect on porphyrinopathy of isonicotinic acid hydrazide (INH) with a low concentration of griseofulvin (GF), the two chemicals were given to mice simultaneously. INH was added to drinking water at a concentration of 0.05%. GF was mixed with feed at a concentration of 0.1%. The mice (dd‐y strain) were divided into four groups. Those in group A were fed normally. Group B received only 0.1% GF, group C received only 0.05% INH, and group D received both 0.1% GF and 0.05% INH. The treatment was continued for 13 to 30 weeks. After treatment, the mice were anesthetized and sacrificed. The liver and whole blood were taken for analysis of porphyrins. The results revealed a slight elevation of erythrocytic porphyrins in the groups treated with 0.1% GF or 0.05% INH alone and remarkable abnormalities in the hepatic and erythrocytic porphyrin levels of the group simultaneously treated with both chemicals. These results show that INH itself may have a small potential for the induction of porphyric abnormalities, and that the administration of INH with a low concentration of GF induces marked porphyrinopathy in dd‐y strain m