AIM: To investigate the effects of short- and long-term intraperitoneal thymoquinone administration on basilar artery morphometry and cerebral perfusion in a subarachnoid haemorrhage (SAH) model. MATERIAL and METHODS: Forty-two adult male Wistar rats were randomly allocated to seven experimental groups, including control, sham, untreated SAH, and SAH groups treated with vehicle or thymoquinone (10 mg/kg/day) for 3 or 8 days. SAH was induced by autologous blood injection into the cisterna magna, and animals were sacrificed on days 3 or 8. Basilar artery luminal and tunica media areas were assessed using unbiased stereological methods. Cerebral blood flow was measured by laser Doppler flowmetry, and neurological function was evaluated as a secondary outcome. RESULTS: Basilar artery luminal area differed significantly among experimental groups (p<0.001), with pronounced luminal narrowing in vehicle-treated SAH animals during the delayed phase (day 8). Prolonged thymoquinone administration significantly attenuated delayed luminal narrowing, whereas short-term treatment did not produce a measurable morphometric effect. Long-term treatment was associated with partial preservation of cerebral blood flow. No significant differences were observed in basilar artery tunica media area. Neurological deficit scores differed among groups, with significant impairment in vehicle-treated SAH animals at day 8 compared with both control and thymoquinone-treated SAH animals at the same time point. This study demonstrates duration-dependent vascular effects of thymoquinone in experimental SAH. CONCLUSION: Prolonged intraperitoneal thymoquinone administration exerts time-dependent neurovascular effects in experimental SAH by attenuating delayed basilar artery luminal narrowing and partially preserving cerebral perfusion, highlighting the importance of treatment duration in delayed vascular pathology.