A North Dakota strain of the Colorado potato beetle, Leptinotarsa decemlineata (Say), was reared under both short- (8L:16D) and long-day (17L:7D) conditions. Age-related and pyriproxyfen- (JHA-) induced changes in hemolymph free amino acids and proteins were examined. Under a short-day photoperiod, the total free amino acid concentration in the hemolymph increased gradually up to 20 days of adult life, but the long-day beetles showed marked increases during the first 10 days and then decreased afterwards. Proline, glutamine and valine were the most abundant free amino acids in both sexes of beetles held under either short- or long-day photoregims. JHA treatment of diapausing adults, held under either short- or long-day conditions after treatment, terminated diapause as indicated by re-emergence from the vermiculite, feeding, mating, changes in free amino acid levels, the disappearance of diapause protein 1 and appearance of vitellogenin in the hemolymph. Furthermore, most of the JHA-treated females held under long-day conditions also matured oocytes and oviposited, but those held under short-day conditions did not.