The abdominal ganglion neurosecretory cells responsible for the synthesis and release of two insect neurohormones, cardioacceleratory peptides 1 and 2 (CAP1 and CAP2), from the perivisceral organs (PVOs) have been identified in the tobacco hawkmoth, Manduca sexta. Previous work established the existence of two groups of abdominal ganglion cell bodies with axons projecting to the PVO: four laterally-situated pairs and five pairs lying on the midline (Taghert & Truman, 1982b). Micro-dissection and bioassay of various parts of an abdominal ganglion revealed that CAP activity was greatest in the medial portion of the ganglion, the portion containing the 10 midline neurones. Six of the 10 midline neurosecretory cells, the new midline bilateral (MB) cells, appeared to differentiate post-embryonically, commencing differentiation late in the last larval instar and reaching maturity midway through adult development. The development of the new MB cells was mirrored by the accumulation of CAP activity in the abdominal nerve cord. Not present in measurable amounts in larvae, CAP activity was first detectable a few days after pupation and reached maximal levels midway through adult development. CAP-like bioactivity was collected from the PVO in response to antidromic stimulation of the nerve containing the new MB axons. No CAP-like bioactivity was detected in those preparations in which the new MB axons were severed or in which other nerves were stimulated. Intracellular stimulation of a new MB neurone evoked the release from the PVO of measurable levels of CAP bioactivity. It was shown that this stimulation-evoked, cardioacceleratory activity was sensitive to protease treatment, and was released only from the cell that was stimulated. On the basis of these experiments, it was concluded that the CAPs are synthesized and secreted from the new MB cells.