Two compounds, 9,10-epoxytetrahydroedulan (ET) and viridiflorine s-lactone (VL), were identified as major components from the hairpencils of field-caught males of a danaid butterfly, Euploea mulciber. By contrast, laboratory-reared males entirely lacked VL, but possessed a significant quantity of ET. Various feeding experiments with larvae and indoor adult males strongly suggested that ET is biosynthesized de novo only after eclosion from nutrients ingested during the larval development. Since VL was suspected to be derived from pyrrolizidine alkaloids (PAs) acquired as an adult, tests for feeding response to and oral administration of four PAs (a 4:1 mixture of intermedine/lycopsamine, heliotrine, monocrotaline, and retronecine) were conducted. When the tarsi or proboscis were stimulated with PA solutions, males showed positive feeding responses (proboscis extension and sucking movements) to intermedine/lycopsamine, heliotrine, and retronecine in decreasing order of responsiveness, thereby providing evidence that male adults are endowed with taste receptor(s) specific to PAs on the legs as well as on the proboscis. Differently from gustatory responsiveness, only males fed with intermedine/lycopsamine produced a significant quantity of VL (ca. 35 μg/male), whereas those that ingested heliotrine or monocrotaline hydrochloride produced traces of VL (<0.18 μg/male). Uptake of retronecine did not lead to VL formation at all. In behavioral bioassays to test the attractivity of PAs to males, all individuals tested were attracted exclusively to intermedine/lycopsamine. This shows that certain PA(s) per se serve as attractant(s) for males in locating PA sources, and further suggests that in the field, males will seek particular PA(s) that are indispensable as precursors for the efficient biosynthesis of VL.