7?6 mg/kg bw per day) maximally induced a twofold increase in hepatic porphyrin concentration (lowest effect doses: TCDD, 47 ng/kg bw per day; 3,3?,4,4?,5-PeCB, 3. 2 ?g/kg bw per day; 2,3,3?,4,4?,5-HxCB, 360 mg/kg bw per day). Concomitant administration of TCDD and 3,3?,4,4?,5-PeCB or 2,3,3?,4,4?,5-HxCB resulted in an additional twofold increase in porphyrin concentration. When these compounds were administered with a non-inducing dose of TCDD (33 ng/kg bw per day), however, an 800-fold increase in accumulation of porphyrins was seen, in particular uroporphyrin III and heptacarboxylic porphyrin, in the liver, indicating a strong synergistic action. Furthermore, induced acetanilide 4-hydroxylase.