The work presented concerns hormonal control of crustacean growth and reproduction by eyestalk neuropeptides and the androgenic gland hormone (AGH). The elements of the mandibular organ inhibitory hormone (MO-IH) neurosecretory system are described in larvae and mature crabs, Cancer pagurus and related to other neuropeptides including moult inhibiting hormone, (MIH) and crustacean hyperglycemic hormone, (CHH). Polyclonal antisera raised against HPLC-purified C. pagurus MO-IH and MIH and cRNA probes to MO-IH and MIH mRNA used in immunocytochemical (ICC) and in situ hybridisation (ISH) studies show that MO-IH and MIH are colocalised and coexpressed throughout intermoult. MO-IH and MIH immunoreactive structures are distinct from those containing CHH immunostaining. Similarities between the second messengers of CRH-family neuropeptides have also been observed, and in vitro cGMP production by YO of C. pagurus and Carcinus maenas arising from incubation with MIH, MO-IH and CHH is correlated with inhibition of ecdysteroidogenesis. Analogues of cGMP (and to a lesser extent cAMP) inhibit in vitro ecdysteroidogenesis. Neuropeptides controlling MF synthesis by MO, i.e. MO-IH (C. pagurus) and CHH (C. maenas) stimulate in vitro cGMP production in MO, although the physiological effects of this phenomenon are unclear. MO stimulated in vitro ecdysteroidogenesis by crab Y-organs (YO), although results do not implicate the involvement of MF. The androgenic gland (AG) of C. maenas is a ductless, vascularised endocrine organ controlling sperm formation (spermatogenesis), maturation (spermiogenesis) and formation of secondary sex characteristics. Histological studies and monitoring of the testicular index (T.I.) related changes in AG morphology with that of the testes and moult cycle in C. maenas .. Andrectomy did not result in the development of ovotestes in mature C. maenas, as observed in other decapods, but stimulated rapid atrophy of testicular tissues and reduced testicular protein synthesis. Conversely, eyestalk removal (ESR) stimulated developement of AG and testes, and increased T.I. and protein synthesis. A bioassay was developed to monitor the effect of AG upon gonadal protein synthesis throughout reproductive cycle. Only preitellogenic (stage 0/1) ovaries and non-spermatogenic testes were AG-sensitive stages and a low circulating titre of vitellogenin is critical to AG activity within the ovarian bioassay. Evidence for a proteinaceous AGH in C. maenas is suggested, challenging the assumption that famsylacetone (FA) is the AGH of decapods. ESR stimulated AG hypertrophy and AG-mediated inhibition of ovarian protein synthesis throughout the year ( excepting summer). Results indicate that, in the absence of vitellogenesis inhibiting honnone (VIH) in crabs, CHH may be the eyestalk neuropeptide responsible for regulating AG activity in C. maenas.