Synchronous oscillation of membrane potentials, generated by assemblies of neurons, is a prominent feature in the olfactory systems of many vertebrate and invertebrate species. However, its generation mechanism is still controversial. Biogenic amines play important roles for mammalian olfactory learning and are also implicated in molluscan olfactory learning. Here, we investigated the role of serotonin, a biogenic amine, in the oscillatory dynamics in the procerebrum (PC), the molluscan olfactory center. Serotonin receptor blockers inhibited the spontaneous synchronous oscillatory activity of low frequency (approximately 0.5 Hz) in the PC. This was due to diminishing the periodic slow oscillation of membrane potential in bursting (B) neurons, which are essential neuronal elements for the synchronous oscillation in the PC. On the other hand, serotonin enhanced the amplitude of the slow oscillation in B neurons and subsequently increased the number of spikes in each oscillatory cycle. These results show that the extracellular serotonin level regulates the oscillation amplitude in B neurons and thus serotonin may be called an oscillation generator in the PC. Although nitric oxide (NO) is known to also be a crucial factor for generating the PC oscillatory activity and setting the PC oscillation frequency, the present study showed that NO only regulates the oscillation frequency in B neurons but could not increase the spikes in each oscillatory cycle. These results suggest complementary regulation of the PC oscillatory activity: NO determines the probability of occurrence of slow potentials in B neurons, whereas serotonin regulates the amplitude in each cycle of the oscillatory activity in B neurons.