The presence of allatostatin-like immunoreactivity (ALI) has been demonstrated throughout the nervous system, gut, and peripheral tissues of unfed fifth instar and adult Rhodnius prolixus. ALI in apparent neurosecretory cells of the brain, suboesophageal ganglion, and mesothoracic ganglionic mass, as well as in midgut endocrine-like cells, suggests that Rhodnius allatostatins may act as neurohormones/hormones. The presence of ALI in possible interneurons and areas of neuropile throughout the central nervous system (CNS) also suggests roles as neuromodulators and/or neurotransmitters.The inhibitory properties of Diploptera punctata AST 7 (Dippu-AST 7) has been demonstrated on the Rhodnius hindgut and dorsal vessel. Because of the low concentrations capable of inhibiting both spontaneous (10-12 M) and leucokinin 1 (LK 1)-induced contractions (10-10 M--10-9 M) of the hindgut, as well as spontaneous (10-12 M) and serotonin-induced (10-9 M) dorsal vessel contractions, it is suggested that AST-like peptides in Rhodnius act as neurohormones on these tissues.