A general model for heat exchange, comprising the major models in the literature, was developed. Temperature changes as a function of space and time were determined in six resting humans (age 32.7 +/- 4.5 yr) during temperature transients by magnetic resonance imaging (MRI), so that the exact solution of the model was obtained. These results allowed application of the model, e.g., the analysis of transient muscle heat flow changes, which could not be assessed by previous steady-state analyses. A microthermocouple was inserted in muscle vastus lateralis 2-3 cm below the skin surface. The measured temperature was used for calibrating the pixel intensity of a temporal series of transaxial magnetic resonance images obtained with a spin echo sequence around the microthermocouple position. After muscle temperature was increased by immersion in a controlled water bath, MRI acquisition was performed while muscle temperature was decreasing. Temperature maps relative to space and time inside a homogeneous region of interest were reconstructed by neural networks, showing specific temperature patterns. Subsequently calculated heat flows (with negative sign) appeared to increase linearly as temperature decreased, until a maximum was attained at a critical temperature, below which dramatic consistent heat flow changes were found. In conclusion, MRI is indeed a powerful technique, useful to study the determinants of muscle temperature and heat flow changes in space and time.