Dry-weight changes in the midgut, abdominal fat body, flight muscles, pterothoracic cuticle, and reproductive tract were measured in unfed male Rhodnius prolixus Stål following eclosion and in males fed a single blood meal on day 5 post-ecdysis. Males, which had fed to repletion as fifth instar nymphs, emerged from the nymphal–adult moult with approximately the same quantity of blood solids in the midgut as observed in fully engorged males. Carry-over of the blood-meal from the fifth instar was sufficient to allow the fat body, flight muscles, and pterothoracic cuticle in unfed males to attain over 85% of the weight accretion observed in fed males. The reproductive tract dry weight in unfed adults increased only 60–65% of that in fed adults. Flight-muscle and reproductive-tract development in fed insects reached growth plateaux in 2 and 3 weeks after the final nymphal moult, while cuticle development continued for up to 10 days. It is concluded that R. prolixus males which emerge from fully engorged fifth instars do not require a blood meal in the adult stage to trigger growth processes and tissue development, but if the adult takes at least one blood meal, significant increases in the reproductive-tract dry weight and slight increases in the dry weights of other tissues will occur.