Hypoxia inducible factors (HIFs) are heterodimeric transcription factors that mediate the adaptive response of mammalian cells and tissues to changes in tissue oxygenation. In the present study, we show an age-dependent decline in cortical HIF-1 accumulation and activation of HIF target genes in response to hypoxia. This inducible response is significantly attenuated in 18 month and virtually absent in 24 month F344 rat cerebral cortex. This attenuated HIF-1 response had no effect on mRNA upregulation of HIF-independent genes in the aged cortex. We have provided evidence that this absent HIF-1 response is directly correlated with an increase in the expression of the HIF regulatory enzyme, prolyl 4-hydroxylase (PHD). In addition, our study shows that cortical HIF-2 expression in senescent normoxic controls is also significantly greater than that of younger normoxic controls, despite no difference in HIF-2 mRNA levels. The post-translational regulation of HIF-2 under normoxic conditions seems to be attenuated in the aged rat brain, which is an in vivo demonstration of differential regulation of HIF-1 and HIF-2.