1. Slices of human and rat liver were cryopreserved in 18% dimethyl sulphoxide (DMSO) and subsequently stored in liquid nitrogen for periods up to as long as 6 months. After thawing, the metabolism of testosterone to hydroxylated products and conjugation of 7-hydroxycoumarin were investigated. 2. Rat liver slices stored in liquid nitrogen for 6 months exhibited rates of formation of 7alpha-, 6beta- 16alpha- and 2alpha-hydroxytestosterone, and of androstenedione that did not differ significantly from those observed with fresh slices. 3. No formation of 2alpha-hydroxytestosterone was detected with slices of human liver. However, in contrast with the rat, human slices produced 2beta-hydroxytestosterone. The rates of formation of 7alpha-, 6beta-, 16alpha- and 2beta-hydroxytestosterone and of androstenedione by human liver slices after 6 months of storage in liquid nitrogen were 82, 71, 236, 66 and 92%, respectively, of the corresponding rates by fresh slices. 4. The rates of sulphation and glucuronidation of 7-hydroxycoumarin by slices from rat liver were 97 and 119%, respectively, of the corresponding fresh values after 6 months of storage in liquid nitrogen. 5. 7-Hydroxycoumarin glucuronidation by human liver slices was 53% of the corresponding fresh values after 6 months of storage. However, human slices showed little or no capacity to conjugate 7-hydroxycoumarin with sulphate. 6. It was demonstrated that slices of both human and rat liver can be cryopreserved and stored in liquid nitrogen for at least 6 months without major changes in their rates of metabolism of testosterone to its hydroxylated products and of 7-hydroxycoumarin conjugation. These findings further emphasize that cryopreservation of liver slices can be an effective tool in the use of biological material of limited availability.