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1. Evoked electrical activity was observed in slices from guinea-pig olfactory cortex maintained in vitro. This evoked activity was studied in saline solutions containing various concentrations of Ca²⁺ and Mg²⁺.

2. The evoked potentials recorded from the surface of the prepiriform cortex comprised a negative wave (N-wave) of about 10-15 msec duration upon which a variable number of short duration positive notches were superimposed. The N-wave was identified as a population excitatory post-synaptic potential (EPSP) of the olfactory cortex neurones and the positive notches were identified with synchronous discharge of cortical neurones.

3. The relation between the EPSP amplitude and saline Ca²⁺ concentration was sigmoid. The N-wave amplitude (EPSP) was proportional to [Ca²⁺]_oⁿ. When saline Ca²⁺ was between 0·4 and 1 mM n averaged 2·5 (range 1·5-4·3).

4. The EPSP amplitude was reduced by increasing saline [Mg²⁺]. High Mg²⁺ salines (about 10 mM) abolished the EPSP.

5. Both Ca²⁺ and Mg²⁺ in high concentrations depressed the positive notches.

6. Salines containing little or no Ca²⁺ and Mg²⁺ caused the preparation to show signs of hyperexcitability. Under these conditions the evoked potentials were very variable and unstable. `Ca²⁺-free' salines caused a reversible loss of excitability.

7. The results indicated that Ca²⁺ and Mg²⁺ had antagonistic effects on the release of the transmitter substance(s). Increased Ca²⁺ concentrations increased the output of transmitter, increased Mg²⁺ concentrations reduced the output of transmitter. Both Ca²⁺ and Mg²⁺ in increasing concentration increased the threshold for action potential generation.