Six pyrokinins, members of a widely distributed neuropeptide family in insects (FXPRLamides), have been identified from the American cockroach. Five of these peptides, Pea-PK-1-5, were tested in different myotropic bioassays, including hyperneural muscle, hindgut, foregut and oviduct. Among these muscles, the hyperneural muscle exhibited the highest sensitivity to pyrokinin applications. The efficacy of the different pyrokinins differed dramatically. No muscle specific effectiveness was obtained; the ranking order in all muscle assays was as follows: PK-1>PK-4>PK-3>PK-2>PK-5. Testing of synthetic analogs revealed the importance of the amino acid at the variable -4 position of the C-terminus. PK-5, the only one of the five tested peptides which is stored in abdominal perisympathetic organs, has probably no myotropic function at all. This is further evidence that these neurohemal release sites are not necessary to compensate the open circulatory system of insects but have rather specific functions which are totally unknown as yet.