Recently, minimal and non-invasive tools for organismic physiology like MR imaging and spectroscopy (NMR) became more and more available in zoological physiology. These techniques have opened a broad window of applications in comparative physiology. For example, we could show that in vivo MR imaging and spectroscopy techniques adequately monitor the effects of long-term anoxia (up to 11 days) on circulatory performance, cellular energy metabolism and acid-base regulation in freshwater turtles (Trachemys scripta). In another project, tissue oxygenation and blood flow were studied together with energy metabolism and acid-base regulation in ground squirrels during hibernatation and arousal. African lungfishes (Protopterus spp.) undergo aestivation during dry periods. Recently, P. spp. were successfully brought into aestivation after building a dried mucous cocoon inside a plastic container (Chew et al. 2004). This set-up was used to investigate aestivating lungfish under normoxia and hypoxia in a MRI scanner by monitoring high-energy phosphates and intracellular pH through in vivo 31P-NMR spectroscopy over a time period of 12 days. Large changes occurred in phosphodiester concentrations during transition to aestivation, indicating dramatic changes in cellular membrane properties.Chew et al. (2004). J. Exp. Biol. 207, 777-786.