Sumatriptan has been among the top choices in the management of migraine headaches. The association between migraine and epilepsy highlights the possible effect of sumatriptan on seizures. In this regard, we investigated sumatriptan effects on PTZ-induced seizures thresholds and delineated the modulatory role of 5-HT1B/D receptors and NOS/NO pathway. Our data revealed the anti-convulsant effects of lower doses of sumatriptan, and pro-convulsant effects of higher doses of sumatriptan. GR 127935, a selective 5-HT1B/D antagonist, could abolish the sumatriptan anti-convulsant effects, but it was ineffective against the sumatriptan pro-convulsant effects. Serotonin depletion by consecutive administration of p-CPA, a selective irreversible inhibitor of tryptophan hydroxylase, could not affect the anti-convulsant effects of sumatriptan. The anti-convulsant effects of sumatriptan was potentiated by L-NAME, a non-selective NOS inhibitor, 7-NI, a selective nNOS inhibitor, but not AG, an iNOS inhibitor. It was also neutralized by L-ARG, a NO precursor. The pro-convulsant effects of sumatriptan were blocked by L-NAME and AG, but not 7-NI. It was also potentiated by L-ARG. Our data revealed that anti-convulsive effects of sumatriptan is mediated by interaction between non-serotonergic 5-HT1B/D receptors and nNOS/NO pathway. Besides, the pro-convulsive effect of sumatriptan is mediated by iNOS/NO pathway independent of 5-HT1B/D receptors. For the first time, this study reported the biphasic effect of sumatriptan on an animal model of GCS and its modulatory pathways.