This study explores the role of ERK activation in regulating G1 and S-G2/M delays during hyperoxia. We demonstrate here that exposing A549 human alveolar type 2 adenocarcinoma cells to hyperoxia (95% O2) for 0.5–24 h time-dependently increases phospho-ERK, phospho-p53Ser15, p53, and p21CIP1 protein levels. Decreasing phospho-ERK with the pharmacological inhibitors, PD98059 and U0126, markedly suppresses hyperoxia-stimulated phospho-p53Ser15, p53, and p21CIP1, and also restores the hyperoxia-reduced kinase activities of cyclin D1/E1-Cdks. Our results suggest that ERK activation during hyperoxia contributes to the p53/p21-mediated G1 checkpoint. However, inhibition of ERK signaling during hyperoxia further delays S-phase entry and progression. Hyperoxia induces significant expression of cyclin A/B1 and translocation of cyclin A into nuclei while marginally decreasing cyclin A/B1-Cdks kinase activities, which may be related to nuclear association with p21. Interestingly, inhibition of ERK signaling markedly suppresses the elevation of cyclin A/B1 proteins and cyclin A/B1-Cdks kinase activities during hyperoxia. Taken together, the results presented here suggest that hyperoxia-activated ERK acts upstream of p53 and p21 to suppress G1-Cdk activities; however, it is also required for induction of cyclin A/B1 and maintenance of cyclin A/B1-Cdk activities that oppose delays in S-phase entry and progression.