We examined cerebral arteriolar structure and autoregulation of cerebral blood flow (CBF) in control (n = 8), sham-operated (n = 8), pinealectomized (n = 10), and pinealectomized plus melatonin-treated (0.51 ± 0.01 mg · kg¹ · day¹ in drinking water, n = 9) young Wistar rats. The lower limit of CBF autoregulation (LLCBF) was determined by measurement of CBF (in arbitrary units, laser Doppler) during stepwise hypotensive hemorrhage; the arteriolar internal diameter (ID; in µm, cranial window) was also measured. Measurements of ID were repeated during a second stepwise hypotension after smooth muscle cell deactivation (67 mmol/l EDTA). The cross-sectional area (CSA) was measured by histometry. CSA and EDTA-induced vasodilatation decreased after pinealectomy (517 ± 21 vs. 819 ± 40 µm² in sham and 829 ± 55 µm² in control, P < 0.05, and 81 ± 4 vs. 102 ± 5 µm in sham and 104 ± 4 µm in control, P < 0.05, respectively) and were restored by melatonin (924 ± 39 µm² and 102 ± 5 µm, respectively). These results suggest that melatonin deprival makes the arteriolar wall thinner and stiffer. However, these changes had little effect on LLCBF. In conclusion, pinealectomy of young rats induces atrophy and decreases distensibility of the cerebral arteriolar wall; these effects are prevented by melatonin. They do not modify LLCBF.