The pentapeptide proctolin, originally identified in the cockroach, has been shown to be widely distributed in many insects and to have a broad range of physiological functions. In the oviduct of the locust, Locusta migratoria, proctolin's role as a neurotransmitter/neuromodulator has been well documented; however, a neurohormonal role in the locust is less certain. This review will examine the various roles of proctolin in locust oviduct contraction and will present evidence that a substance chromatographically, immunologically and physiologically indistinguishable from proctolin is present in the hemolymph of the locust, L. migratoria. This material is concentrated in the plasma, rather than the hemocytes, and is present at concentrations ranging from 0.1 to 0.2nM. This review extends the role of proctolin in insects, and suggests that proctolin may play a neurohormonal role in the locust.