The role of some neuromodulators and neurotransmitters in the functioning of molluskan cerebral neurons and in their metabolic changes during hibernation has been considered. The cerebral ganglion of mollusks is a center for the integration of different inputs from the sensory areas of the head and for the generation of motor command impulses. During hibernation, animals are deprived of many external sensory stimuli and do not have locomotion and feeding. Immunocytochemistry for bioactive peptides (BAPs), such as SP (Substance P), CCK8 (Cholecystokinin 8/Gastrin), CGRP (Calcitonin-Gene-Related Peptide) and ET (Endothelin), and serotonin was performed on cerebral ganglia of active and hibernating Helix aspersa. The distribution of the immunopositivity was analyzed in different cell-containing areas (procerebrum, mesocerebrum, metacerebrum) and in the neuropiles. With all the antibodies raised against peptides, we observed that only a few neurons, mainly of small and medium size, had immunopositivity during the period of activity, the patterns of distribution being quite similar to those previously described in Helix or other gastropods. Fibers and varicosities with BAP immunopositivity were found in the procerebral and central neuropiles and sometimes around neurons. Serotonin-immunopositive neurons, including the giant neuron, were observed in the metacerebrum; numerous fibers and varicosities immunopositive for serotonin were present in the neuropile areas. In hibernating snails, the number of fibers with BAP and serotonin immunopositivity decreased in several areas of the neuropiles. Moreover, an increased number of neurons of the metacerebrum (two-to four-fold) and mesocerebrum (8- to 28-fold) had BAP-like immunopositivity, and the intensity of the immunoreaction for serotonin of the metacerebral neurons was also higher than in the active snails. These results are discussed, taking into account two hypotheses. The first hypothesis assumes that the increased immunocytochemical staining was really linked to accumulation of BAPs and serotonin. The second hypothesis considers that the antibodies for BAPs recognized a preprotein, the synthesis of BAPs being completed during the active period only. Both the hypotheses account for the co-occurrence and co-localization of two or ore peptides and serotonin and stress that the hibernation condition is of interest for studies on the actual function of single neurons in the cerebral ganglia. Finally, the data are consistent with the changes recently found in other markers of the morphological and functional activity of neurons, demonstrating that the neuromodulation and the neurotransmission are slowed during hibernation.