We have studied the dynamic changes in tissue vasculature following the inhalation of hyperoxic gasses in rodents as a model for optical breast cancer detection. We have used a CW apparatus to measure the near infrared (NIR) optical properties of animal models immersed in a liquid tissue phantom. By looking at the transmission of different wavelengths in the NIR, we were able to qualitatively observe changes in blood oxygenation following the inhalation of different mixtures of CO2 and O2. These changes enhanced the image contrast between cancerous tissue and normal tissues of the rodents. The oxygenation dynamics of the tumors, following inhalation of the hyperoxic gases, exhibited differences from surrounding tissues in both the magnitude of the observed signal change and the dynamic response.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.