AbstractThe endocrine regulation of molting was investigated in the crabGecarcinus lateralisby treating tissues from anecdysial (intermolt) animals with the arthropod molting hormone 20‐hydroxyecdysone (20HE) in vitro and observing changes in the incorporation of [35S]‐methionine into proteins. Although total protein synthesis was not affected by levels of 20HE ranging from 10−9to 10−5M, synthesis of five out of 30 protein bands was significantly stimulated at hormone concentrations of 10−9−10−7M, the latter characteristic of proecdysis. Increases were as large as eightfold. Synthesis was not significantly inhibited for any of the integumentary proteins. Treatment of integumentary tissues with 10−5M 20HE (approx. 100 times greater than the highest endogenous concentration) had less effect than physiological concentrations. The dose response varied substantially among integumentary proteins. In some instances, the effects were similar to changes in synthesis that occur in vivo during specific stages of proecdysis (Stringfellow and Skinner, Dev. Biol.,128: 97–110, '88). Midgut gland (hepatopancreas) was also treated with 20HE in vitro. Treatment with 20HE significantly inhibited total protein synthesis in tissue from males, but not in that from females. This treatment inhibited the synthesis of a number of the 32 protein bands examined in both sexes. Synthesis of six protein bands was significantly inhibited in males, while synthesis of three different bands was significantly inhibited in females. This inhibitory action of 20HE is consistent with the atrophy of the midgut gland that occurs during proecdysis (Gibson and Barker, Oceanogr. Mar. Biol. Annu. Rev.