Microenvironmental factors have been demonstrated to play a pivotal role in the selection of neoplastic cell subpopulations expressing more malignant phenotypes and contributing to the progression of localized and metastatic disease. Very low levels of O2 (< 10 mmHg) has been well described in many solid tumours (including prostate cancer) and the extent of hypoxia has been demonstrated to represent an independent marker of a poor prognosis for patients with various types of cancers. Tumour hypoxia contributes to numerous adaptive phenotypes including increased invasion and metastasis, as well as evasion of immune cell surveillance increased resistance to radiotherapy and chemotherapy. Although cellular adaptive responses to hypoxia are likely mediated by various mechanisms, our previous preclinical studies suggest that decreased nitric oxide (NO)-dependent signalling plays a significant role in this progression of a malignant phenotype.