Abstract Butterflies of the tribe Heliconiini (Nymphalidae) are specialist herbivores in which the larvae utilize the plant family Passifloraceae. Their adaptation to their host plant's toxins was examined by analyzing retention and excretion of three Passiflora alkaloids: norharman, harman and harmine. Three species of adult Heliconiines were analyzed for the presence of these β-carboline alkaloids. The butterflies were found to sequester a substantial proportion of the alkaloids ingested by the larvae. Adults of Spodoptera exiqua (Hubner) (Noctuidae) raised as larvae on an artificial diet containing the β-carboline, harman, were also analyzed and they were found to eliminate harman almost completely. Excretion studies, using isolated Malpighian tubules from larvae of Heliconius ismenius clarescens (Heliconiini), identified concentrations of norharman which reversibly inhibit fluid secretion. Norharman appears to be transported in Malpighian tubules by a saturable system.