Nitrous oxide antagonizes CNS stimulation by laudanosine in mice. We have investigated whether nitrous oxide antagonizes or augments the CNS stimulant action of laudanosine in mice by comparing the mean convulsive doses (CD50 (SE] of a control group and those following pretreatment with 65% nitrous oxide in oxygen for 20 and 180 min. Nitrous oxide significantly increased CD50 from 46.8 (1.4) mg kg-1 of control to 57.3 (1.3) mg kg-1 at 20 min and 53.5 (1.7) mg kg-1 at 180 min. The attenuation of the effect of nitrous oxide at 180 min, suggestive of possible partial drug tolerance, was not statistically significant. These findings indicate that nitrous oxide antagonizes the CNS stimulating action of laudanosine. 