The effects of combined administration of ochratoxin A (OA) and aflatoxin B1 (AFB1) on the occurrence and the levels of residues of mycotoxins in poultry have been investigated. Male broilers and laying hens were fed from 14 days old with standard diets contaminated with 50 micrograms/kg OA and 50 micrograms/kg AFB1. Two groups of broilers and hens were withdrawn from contaminated feed at 37 and 88 days, respectively. At the time of sacrifice no significant lesions were found. Residues were compared with those found after administration of either toxin alone in former trials. Combined treatment resulted in higher content of OA in broiler livers (40 versus 5.0 micrograms/kg) and, to a lesser extent, in kidneys and skin, and of AFB1 in broiler liver and kidney (0.15 versus 0.02 microgram/kg and 0.40 versus 0.05 microgram/kg respectively). Laying hens showed smaller differences (0.20 versus 0.10 microgram/kg in liver and 0.32 versus 0.08 in kidneys). Withdrawal from treatment led to the almost complete disappearance of OA residues in broilers and in hens. These results show a synergistic effect of OA and AFB1, particularly in broilers.