Changes in both synthesis rate and degradation rate of ornithine decarboxylase (ODC) were pursued in primary cultures of adult rat hepatocytes during the process of ODC induction caused by asparagine and glucagon and also during the process of rapid ODC decay caused by putrescine. The synthesis rate of ODC was determined by [35S]methionine incorporation into the enzyme, which was separated afterwards by immunoprecipitation and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The degradation rate of ODC was determined by following the decay of prelabeled ODC. The enzyme induction caused by asparagine (10 mM) and glucagon (1 microM) was due both to an increase in the synthesis rate and to a decrease in the degradation rate. Addition of 10 mM putrescine caused a rapid decay of ODC activity, which was faster than ODC decay in the presence of cycloheximide. This rapid decay in ODC activity was accompanied by slightly slower decay in ODC protein, which was due both to partial suppression of ODC synthesis and to several fold acceleration of ODC degradation.