Prothoracicotropic hormone (PTTH) is an insect brain neuropeptide that is a primary factor regulating an insect development. Curtailment of its release is thought to be responsible for the pupal diapause of tobacco hornworm, Manduca sexta. The cell synthesizing and secreting the PTTH has been identified as a pair of neurosecretory cells in the pars lateralis on each brain hemisphere. Using intracellular recording techniques, we have demonstrated electrical properties of the PTTH cells in different physiological status, i.e., diapausing and developing pupae. In diapausing pupae, they showed threshold value increasing and input resistance decreasing with the progress of diapausing state, indicating that they were getting unexcitable. Spontaneous action potentials and excitatory postsynaptic potentials (EPSPs) were rarely observed in deeply diapausing state. Non-diapausing PTTH cells were almost silent except day-2, showing rather constant values of electrical properties. On day-2, a significant proportion of the cells had spontaneous action potentials, showing less negative membrane potential values than inactive cells. Exclusively inhibitory postsynaptic potentials (IPSPs) were observed in significant numbers of the cells during the period from day-2 to day-5. On the basis of the results obtained, we proposed a working hypothesis that electrical activities of the PTTH cell may be primarily regulated by its membrane properties which are further modulated by the synaptic mediation.