AbstractThe abdominal and pedal‐pleural ganglia ofAplysia californicawere maintained in organ culture for periods up to 45 days. Neurophysiological examination of abdominal ganglia indicated that synaptic activity could be evoked in ganglia cultured for 21 days and action potentials and spontaneous postsynaptic potentials persisted for 45 days. The effects of long‐term culture on neurotransmitter metabolism were studied by measuring the activities of ChAc, AChE, AAD, and COMT in both cultured abdominal and pedal‐pleural ganglia. G‐6‐PD and CarnAc activities were also measured. In general, the stabilities of the enzymes in abdominal ganglia toin vitroconditions were similar to those in pedal‐pleural ganglia. None of the enzymes studied were markedly affected by culture conditions for at least fourteen days. After this time, enzymes involved in neurotransmitter synthesis, AAD and ChAc, exhibited fairly abrupt decreases in activity. The degradative enzymes studied, AChE and COMT, showed more gradual decreases in activity. The changes occurring in the neurotransmitter metabolizing enzymes after prolonged periods of time in culture are discussed in relation to the neurophysiological changes which we