Data are presented demonstrating that adenylate kinase (AK; EC 2.7.4.3) is an oncodevelopmental enzyme in the prostate of Copenhagen rats. We selected the Dunning tumor (dorsal rat prostate) as a model system because it most nearly approximates the human pathology. Four sublines of the tumor (R3327-H, R3327-AT, MAT Lu, and MAT LyLu) were studied. The tumor sublines were maintained as solid tumors in syngeneic rats and as monolayers in tissue culture. AK activity appeared in conjunction with malignant transformation of the dorsal prostate. We also determined the normal developmental enzyme pattern: AK was present in prostates of newborns, but was undetectable in prostates of adults. However, AK increased after castration. Therefore, we propose AK as a potential oncofetal tumor marker in prostatic cancer.