An immunoblotting technique was used to study the forms of ornithine decarboxylase (ODC) present in androgen-induced mouse kidney. Two forms were detected which differed slightly in isoelectric point but not in subunit MW (.apprx. 55,000). Both forms were enzymatically active and could be labeled by reaction with radioactive .alpha.-(difluoromethyl)-ornithine, an enzyme-activated irreversible inhibitor. On storage of crude kidney homogenates or partially purified preparations of ODC, the enzyme protein was degraded to a smaller size (MW .apprx. 53,000) without substantial loss of enzyme activity. The synthesis and degradation of ODC protein were studied by labeling the protein by i.p. injection of [35S]methionine and immunoprecipitation using both monoclonal and polyclonal antibodies. The fraction of total protein synthesis represented by renal ODC was increased at least 25-fold by testosterone treatment of female mice and was .apprx. 1.1% in the fully induced androgen-treated female. Both forms of the enzyme were rapidly labeled in vivo, and the immunoprecipitable ODC protein was almost completely lost after 4-h exposure to cycloheximide, confirming directly the very rapid turnover of this enzyme. Treatment with 1,3-diaminopropane which is known to cause a great reduction in ODC activity did not greatly selectively inhibit the synthesis of the enzyme. However, 1,3-diaminopropane did produce an increase in the rate of degradation of ODC and a general reduction in protein synthesis. These 2 factors, therefore, appear to be responsible for the loss of ODC activity and protein in response to 1,3-diaminopropane.