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1. The effects of orally administered aqueous lyophilized extract of the leaves of Rhazya stricta (2, 4 & 8 g/kg) on aspects of nervous system function were investigated in mice. 2. In three antinociceptive tests (hot plate, abdominal constriction, and warm water tail flick tests), the extract exhibited dose-dependent and significant antinociceptive activity. Naloxone was ineffective in antagonizing the analgesic effect of Rhazya stricta on tail-flick and abdominal constriction tests, possibly indicating that this effect occurs via non-opiate pathways. 3.Pretreatment of mice with the xenobiotic metabolizing enzymes inhibitor cimetidine (50 mg/kg) did not significantly alter the antinociceptive action of the extract, indicating that the effect is probably due to the parent compound(s) present in the extract and not to metabolites thereof. 4. Rhazya stricta produce dose-dependent sedation, decreased motor activity, and impaired motor control. Time spent on a rotarod treadmill was significantly decreased after treatment with the extract. 5. Rhazya stricta extract (8 g/kg) produced a degree of sedation comparable to that produced by diazepam (5-10 mg/kg), and also significantly increased the reaction time of the tail-flick test, an action which was not produced by diazepam. 6. Administration of R. stricta extract potentiated pentobarbitone sleeping time in a dose dependent manner. The extract did not significantly antagonize picrotoxin induced convulsions. The extract (4 and 8 g/kg) significantly decreased the rectal temperature of normothermic and hyperthermic mice. 7. Pretreatment with R. stricta extract (8 g/kg) completely prevented the occurrence of aggressive behaviour in male mice. 8. It is concluded that the crude extract of R. stricta has central nervous system depressant properties.

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