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Homogenates from various tissues of the desert locust, Schistocerca gregaria, produced identical patterns of breakdown products when incubated with the neuropeptide adipokinetic hormone I (AKH I). Comparison of disappearance of AKH I from the incubation medium of semi-isolated and isolated Malpighian tubules (MTs) and fat body pieces indicates that both tissues contain substantial capacities to take up and/or breakdown peptide. While the MTs may remove peptide circulating in the haemolymph, the fat body may metabolize peptide which binds to cell surface receptors. Locusta migratoria and Periplaneta americana MTs contain the same complement of degrading enzymes since they produced identical primary products. The initial proteolytic attack on AKH I took place through a post-proline cleaving enzyme (PPCE, residue 6). The use of the synthetic substrate benzyloxycarbonyl-Gly-Pro-7-amido-4-methyl-coumarin confirmed the presence of PPCE. Adipokinetic hormone II from S. gregaria was cleaved between phenylalanine and serine (residues 4 and 5) indicating the action of a chymotrypsin-like endopeptidase. The synthetic substrates glutaryl-Phe-7AMC and N-succinyl-Ala-Phe-Lys-7AMC were not cleaved by MT homogenates; however, such homogenates degraded a tyrosine-containing substrate, N-benzoyl-Tyr ethyl ester. Chymotrypsin from bovine pancreas cleaved all the above substrates.