Oxygen is a potent cerebral vasoconstrictor, but excessive exposure to hyperbaric oxygen (HBO₂) can reverse this vasoconstriction by stimulating brain nitric oxide (NO) production, which increases cerebral blood flow (CBF)-a predictor of O₂ convulsions. We tested the hypothesis that phosphodiesterase-5 (PDE-5) blockers, specifically sildenafil and tadalafil, increase CBF in HBO₂ and accelerate seizure development. To estimate changes in cerebrovascular responses to hyperoxia, CBF was measured by hydrogen clearance in anesthetized rats, either controls or those pretreated with one of these blockers, with the NO inhibitor N-nitro-L-arginine methyl ester (L-NAME), with the NO donor S-nitroso-N- acetylpenicillamine (SNAP); or a blocker combined with L-NAME. Animals were exposed to 30% O₂ at 1 ATA ('air') or to 100% O₂ at 4 or 6 ATA. EEG spikes indicated CNS oxygen toxicity. The effects of PDE-5 blockade varied as a positive function of ambient PO₂. In air, CBF did not increase significantly, except after pretreatment with SNAP. However at 6 ATA O₂, mean values for CBF increased and for seizure latency decreased, both significantly; pretreatment with L-NAME abolished these effects. Conscious rats treated with sildenafil before HBO₂ were also more susceptible to CNS O₂ toxicity, as demonstrated by significantly shortened convulsive latency. Decreases in rCBF reflect net vasoconstriction in the brain regions studied, since mean arterial pressures remained constant or increased throughout. Thus, PDE-5 blockers oppose the protective vasoconstriction that is the initial response to hyperbaric hyperoxia, decreasing the safety of HBO₂ by hastening onset of CNS O₂ toxicity.