We have shown that loss of PKC-epsilon () limits the magnitude of acute hypoxic pulmonary vasoconstriction (HPV) in the mouse. Based on this finding we hypothesized that loss of PKC- would decrease the contractile and/or structural response of the murine pulmonary circulation to chronic hypoxia (Hx). However, the pattern of lung vascular responses to chronic Hx may or may not be predicted by the acute HPV response. Adult PKC- wild type (PKC-^+/+), heterozygous null (PKC-^+/-) and homozygous null (PKC-^-/-) mice were exposed to normoxia or Hx for 5wks. Interestingly, PKC-^-/- mice actually had a much greater increase in right ventricular systolic pressure (RVSP; an index of pulmonary artery [PA] pressure), RV mass and hematocrit in response to chronic Hx than PKC-^+/+ mice. In contrast to the augmented PA pressure and RV hypertrophy (RVH), the pulmonary vascular remodeling was increased less than expected (i.e. equal to PKC-^+/+ mice) in both the proximal (medial thickness) and distal (muscularization) PKC-^-/- pulmonary vasculature. The contribution of increased vascular tone to the PHTN seen in the PKC- null mice was then assessed in additional mice by measuring the acute vasodilator response to nitric oxide (NO) at the end of the treatment period. Acute inhalation of NO reversed the increased PA pressure in hypoxic PKC-^-/- mice, implying that the exaggerated PHTN may be due to a relative deficiency in nitric oxide synthase (NOS). Despite the higher PA pressure, chronic Hx stimulated less of an increase in lung endothelial [e] and inducible [i] NOS expression in PKC-^-/- than PKC-^+/+ mice. In contrast, expression of nNOS in PKC-^+/+ mice decreased in response to chronic Hx while lung levels in PKC-^-/- mice remained unchanged. In summary, loss of PKC- results in increased vascular tone, but not pulmonary vascular remodeling in response to chronic Hx. Blunting of Hx-induced eNOS and iNOS expression may contribute to the increased vascular tone. PKC- appears to be an important signaling intermediate in the hypoxic regulation of each NOS isoform.