 PF-00572778, a CRH antagonist, is expected to attenuate adrenocorticotropin ACTH and cortisol responses to naloxone by blocking the effect of the CRH increases induced by naloxone at the postsynaptic receptors. Demonstration of a statistically significant attenuation of naloxone induced increases in cortisol andor ACTH concentrations by PF-00572778 compared to placebo would thus constitute proof of mechanism for the compound. Therefore, this study is to evaluate pharmacodynamic effects of PF-00572778 following naloxone challenge in healthy subjects. 