We have examined changes of postsynaptic potentials and of pattern activity of the identified silent and oscillatory snail neurones in Helix pomatia during conditioning. Local changes of EPSP or IPSP have been recorded during association following the first stimulus in the silent cells, whereas spike discharges could be observed in response to the 2nd stimulus. In the oscillatory neurones changes of pattern activity have been recorded following the 2nd stimulus, while the first stimulus proved to be ineffective. The formation of temporary connections of snail neurones seemed to be a specific phenomenon, because it was necessary to pair stimuli of different inputs for the development of these modifications. These plastic changes seemed to depend on the interstimulus as well as on the intertrial intervals. Our experimental data underline the probable role of the stimulus parameters and of the electrical properties of neurones during the formation of learned neuronal responses.