The present study tests the hypothesis that chronic hypoxia enhances reactivity to NO through age-dependent increases in sGC and PKG activity. In term fetal and adult ovine carotids, chronic hypoxia had no significant effect on mRNA levels for the beta-1 subunit of sGC, but depressed sGC abundance by 16% in fetal and 52% in adult arteries, through possible depression of rates of mRNA translation (16% in fetal and 52% in adult) and/or increased protein turnover. Chronic hypoxia also depressed the catalytic activity of sGC, but only in fetal arteries (63%). Total sGC activity was reduced by chronic hypoxia in both fetal (69%) and adult (37%) carotid homogenates but this effect was not observed in intact arteries when sGC activity was measured by timed accumulation of cGMP. In intact arteries treated with 300 µM IBMX, chronic hypoxia dramatically enhanced sGC activity in fetal (186%) but not adult (89%) arteries. This latter observation suggests that homogenization either removed an sGC activator, released an sGC inhibitor, or altered the phosphorylation state of the enzyme resulting in reduced activity. In the absence of IBMX, chronic hypoxia had no significant effect on rates of cGMP accumulation. Chronic hypoxia also depressed the ability of the cGMP analogue, 8-pCPT-cGMP, to promote vasorelaxation in both fetal (8%) and adult (12%) arteries. Together, these results emphasize the fact that intact and homogenized artery studies of soluble guanylate cyclase activity do not always yield equivalent results. The results further suggest that enhancement of reactivity to NO by chronic hypoxia must occur upstream of PKG and can only be possible if changes in cGMP occurred in functional compartments that afforded either temporal or chemical protection to the actions of PDE. The range and age-dependence of hypoxic effects observed also suggests that some responses to hypoxia must be compensatory and homeostatic, with reactivity to NO as the primary regulated variable.