The first phase of metabolism of foreign compounds (xenobiotics), involving metabolic activation. These reactions occur mainly in the liver but also in the small intestine and lungs and comprise the microsomal or mixed function oxidase system, NADPH- dependent enzymes and cytochrome P450 proteins. Generally regarded as detoxication reactions, but may in fact convert inactive precursors into metabolically active compounds, and be involved in activation of precursors to carcinogens.