Two pyrrolizidine alkaloids and one pyrrolizidine alkaloid-N-oxide were incubated with microsomal preparations from humans, rat and avocado and the product profiles examined. The alkaloids were converted to dehydroretronecine, the putative toxic metabolite, by both rat and human microsomal preparations. In addition, alkaloid-N-oxides, the major detoxication products from pyrrolizidine alkaloids, were also formed. The pyrrolizidine alkaloid-N-oxide was converted to both dehydroretronecine and the parent alkaloid. This suggests that the toxicity of pyrrolizidine alkaloid-N-oxides could be greater than suggested hitherto as a result of conversion to the toxic metabolite via the parent alkaloid. Quantitative differences in the proportions of products formed by the different microsomal preparations may be of significance in the extrapolation of toxicological data from animal models such as the rat to humans.