The structure-activity relations of [His(7)]-corazonin were studied using two different bioassay systems; i.e. inhibitory effect on spinning rate in the silkworm, Bombyx mori, and darkening response in albino nymphs of the migratory locust, Locusta migratoria. Deletion of the N-terminus, shortening of the peptide and single amino acid substitutions reduced activity in a similar manner except for the minimum effective dose in the two insects. The results also revealed that the residues at position 1, 3 and 5 were particularly important for biological activity. Despite the different physiological affects, the two insect species exhibited similar structure-activity relationships, suggesting that they might have similar receptor systems.