Sildenafil, a phosphodiesterase-5 inhibitor is widely used for the treatment of erectile dysfunction. Recently, the FDA approved the use of sildenafil in the therapeutic treatment of pulmonary arterial hypertension. Sildenafil crosses the blood-brain barrier and has been shown to enhance memory. Tremor, rigidity and akinesia are the most common symptoms seen in Parkinson's disease. Fatigue and sexual dysfunction are the other prominent features seen in Parkinson's disease. Interestingly, sildenafil is used therapeutically to treat sexual dysfunction in Parkinson's disease patients. Currently research on Parkinson's disease focuses on developing novel drug therapies for retarding the nigral dopaminergic neurodegeneration. Hence, we investigated the anti-fatigue and neuroprotective effects of sildenafil. In this study, the effect of sildenafil on fatigue was evaluated using forced swim test in mice. Sildenafil had no effect on fatigue as seen by the swim time. With regard to neuroprotective effects, we investigated the effects of sildenafil using two animal models of Parkinson's disease. In this study, 6-hydroxydopamine-lesioned (unilateral) rats and MPTP-treated mice were used as the animal models of Parkinson's disease. 6-Hydroxydopamine-lesioned rats were used to determine the effect of sildenafil on rotational behavior. Ipsilateral or contralateral rotational behavior can indicate the amphetamine-like activity or apomorphine-like activity of sildenafil. Sildenafil did not induce contralateral or ipsilateral rotations in 6-hydroxydopamine-lesioned rats. Sildenafil did not protect against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopamine depletion in the striatum. (C) 2007 Elsevier Inc. All rights reserved.