Abstract: The present invention concerns the design of a method for determining the activity of UCPs and which method can be used to evaluate the ability of different drugs to modify the activity of said proteins. These drugs may be used in the treatment of all diseases and conditions in which changes in thermogenic activity occur. Examples of these are obesity, fever, cachexia etc. Since the activity of the UCPs produces changes in the respiration rate, in the present method the activity of the UCPs is determined by monitoring the disappearance of NADH or NAD(P)H from the reduction in absorption between 300 and 380 nm or in fluorescence from 420-520 nm. In this method, these coenzymes are oxidised by the mitochondria of the yeast Saccharomyces cerevisiae in which different UCPs are expressed in a recombinant manner.