1. The metabolism of a number of xenobiotics has been investigated in isolated viable rat hepatocytes and kidney tubule fragments in suspension. 2. The level of Phase I metabolism was very low in the kidney cells although such cells possess appreciable Phase II metabolic capacity. 3. These findings are discussed in relation to the physiological role of cytochrome P-450 and the conjugating enzymes, and the value of intact cells systems in assessing inter-organ differences in xenobiotic metabolism.