The activities of 5-aminolaevulinate (5-ALA) synthetase and of various microsomat drug-metabolising enzymes have been determined in the livers of rats pretreated with different drugs and chemicals containing the allyl group. Safrole, isosafrole and secobarbital gave rise to slight increases in 5-ALA synthetase, whereas alclophenac and triallyl cyanurate almost doubled the enzyme activity and the known porphyrogenic agents, allylisopropylacetamide (AIA) and allobarbital caused increases of 1.5- and 2.5-fold, respectively. Allobarbital induced the microsomal drug-metabolising enzymes while secobarbital had only a weak effect and alclophenac and triallyl cyanurate had no effect at all. From these results it is suggested that induction of the synthesis of cytochrome P-450 is not rate dependent on the synthesis haem and induction of porphyrin biosynthesis.