Action potentials and synaptic potentials were recordedin vivo from cortical neurons in baby rats aged 20–25 days using a new method based on the ionophore nystatin. Nystain solution was used to fill a standard glass extracellular microelectrode, and became inserted into membranes. Spikes which were initially recorded as extracellular spikes showed increases in amplitude and were transformed into unipolar quasi-intracellular spikes at 0.5–5.0 min after formation of the high-conductance contact. This method allows stable recording of neuronal activity from cells for at least 1h, and provides a good signal-to-noise ratio. The electrode does not puncture the neuron membrane, with the result that experiments do not require any isolation from vibration. Thus, the results obtained demonstrate that the method is highly efficient for recording thein vivo activity of small nerve cellsn.