The present study describes the (xanthine:NAD+ oxidoreductase, EC 1.2.1.37) synthesis and degradation of chick liver xanthine dehydrogenase in vivo and in organ cultures. The results indicate that control of xanthine dehydrogenase activity is mediated by changes in the rate of enzyme synthesis, but that degradation rates are unaffected. The results also suggest that xanthine dehydrogenase synthesis occurs through a previously unreported intermediate. Detected in cultures of liver tissue, this intermediate apparently is not converted into an active enzyme. A model of synthesis and degradation for xanthine dehydrogenase proposes that the synthesis of the enzyme is proportional to messenger RNA and includes an inactive enzyme precursor and a second inactive intermediate prior to degradation. Integrated mathematical solutions describing the concentration of intermediates as a function of time can be found explicitly for simple models. The appendix to this paper extrapolates solutions for one-, two- and three-step models to generate a mathematical solution for an 'n'-step model containing 'n' intermediates. The rate constants in the solutions can be found experimentally.