Nuclear magnetic resonance (NMR) equipment capable of obtaining high resolution in vivo 31P NMR spectra from tumors in humans will soon be commercially available (e.g. from General Electric). However, many important questions about the 31P NMR spectral characteristics of tumors in vivo remain unanswered. It is the purpose of this project to foresee these important questions and to answer them by a logical series of animal studies. In order to do this, we have developed a novel new technique capable of obtaining in vivo 31 P NMR spectra from rat tumors as small as 2-3 mm in diameter. The specific aims of this study are: (1) Determine if there is any correlation between in vivo 31P NMR spectral patterns and tumor growth rate. (2) Determine whether different types of tumors originating from the same tissue give sufficiently distinct in vivo 31P NMR spectra for the latter to be useful in diagnoses. (3) Determine whether there is a daily circadian variation in the intensities of the various 31P NMR spectral lines observed from tumors in vivo. (4) Determine whether there is any variation in the in vivo 31P NMR spectral patterns from mammary tumors which correlate with stage of the estrus cycle. (5) Determine whether there is any major effect on the in vivo 31P NMR spectral patterns from tumors caused by simple dietary changes. The results of these studies will tell how useful in vivo 31P NMR spectra of tumors will be for diagnostic or screening purposes. In addition, these studies may also reveal times during the day or during the estrus cycle when therapy is optimum.