Using whole-cell recordings from antennal-lobe (AL) neurons in vitro and in situ, in semi-intact brain preparations, we have examined membrane properties that contribute to electrical activity exhibited by developing neurons in primary olfactory centers of the brain of the moth, Manduca sexta. This activity is characterized by prolonged periods of membrane depolarization that resemble plateau potentials. The presence of plateau- potential-generating mechanisms was confirmed using a series of tests established by Russell and Hartline (1982). Brief depolarizing current pulses could be used to trigger a plateau state. Once triggered, plateau potentials could be terminated by brief pulses of hyperpolarizing current. Both triggering and terminating of firing states were threshold phenomena, and both conditions resulted in all-or-none responses. Rebound excitation from prolonged hyperpolarizing pulses could also be used to generate plateau potentials in some cells. These neurons were found to express a hyperpolarization-activated inward current. Neither the generation nor the maintenance of plateau potentials was affected by removal of Na⁺ ions from the extracellular medium or by blockade of Na⁺ currents with TTX. However, blocking of Ca²⁺ currents with Cd²⁺ (5x10^-4M) inhibited the generation of plateau potentials, indicating that in Manduca AL neurons plateau potentials depend on Ca²⁺. Examining Ca²⁺ currents in isolation revealed that activation of these currents occurs in the absence of experimentally applied depolarizing stimuli. Our results suggest that this activity underlies the generation of plateau potentials and characteristic bursts of electrical activity in developing AL neurons of M. sexta.