Abstract: The invention provides methods of characterising the mode of action of a test compound by exposing populations of a living system to control and treatment conditions, using an unlabeled and labeled isotope source materials. The measurement of the isotopomer distribution of metabolites reveals the effect of the test compounds on the metabolism in that living systems. Further embodiments include where the living system is a plant, fungus, invertebrate, bacteria or virus, and where the test compound is a screening lead in a pesticide product development program.