We examined the effect of 70% nitrous oxide (N2O) on locomotion and vi
sual-evoked potentials (VEP) in rats. The animals exposed to N2O showe
d an initial decrease of locomotion, followed by development of tolera
nce and unaltered motor activity during N2O withdrawal. Similarly, an
initial decrease of VEP amplitudes was followed by tolerance to N2O. I
n addition, some amplitudes (N2-P3, P3-N3, and N3-P4) exceeded the con
trol values, indicating an increase of neuronal excitability of the vi
sual system during a long lasting exposure to N2O. The increase of VEP
amplitudes was further potentiated by cessation of this gas. The VEP
latencies after initial increase, returned to normal and remained unal
tered during N2O withdrawal suggesting that the speed of neurotransmis
sion is not essentially changed during chronic exposure to N2O. Howeve
r, a significant increase of neuronal excitability during chronic N2O
exposure, which further increased by cessation of N2O, could be of cli
nical importance. Therefore, monitoring of VEP, particularly the ampli
tude values, may significantly improve a detection of altered neuronal
excitability during anaesthesia and drug withdrawal.