Diethylstilbestrol diphosphate which has been in wide use for cancer of the prostate is said to change to active form (stilbestrol) by acid phosphatase. Tubis et al. (1967) suggested probability of scintiscanning of the prostate gland by 131I labeled diethylstilbestrol diphosphate. Yet there has been no literature on this subject. This paper deals with results of animal experiment and clinical studies with this drug. Fifty five μCi of 131I labeled diethylstilbestrol diphosphate for each rat was administered by the tail vein. Specific activity of 131I labeled diethylstilbestrol diphosphate was 67μCi per mg. Each group of this experiment consisted of 3 male rats weighing 110-150g. Radioactivities of each organ (prostate, liver, kidney, spleen, heart, lung, intestine, adrenal and testicle) were determined in different time intervals (5, 15, 30, 60min., 3, 6, 12, 24hr., 2, 3, 5, 7 day) and the relative specific activity (R. S. A.) of each organ was computed. The highest concentration was observed in the liver followed by the kidney, and a relatively long retension was recognized in the liver. The R. S. A. in the prostate was demonstrated to be lower than expected (Table 1). In clinical studies on 10 patients (7 prostatic cancer, 1 prostatitis, 1 bladder tumor and 1 chronic cystitis), 500μCi of 131I labeled diethylstilbestrol diphosphate was administered intravenously. The uptake by each organ was in the same order as described in the rats, and the uptake by prostatic cancer and its metastatic lesion was not so large as expected. The scintigram of the prostate gland was not depicted. In rabbit experiment the prostatic concentration of 131I labeled diethylstilbestrol diphosphate was 0.03-0.04% of the administered dose (56μCi/kg) in 3 hours. Prior administration of non labeled diethylstilbestrol diphosphate (71.4mg/kg) did not alter this radioactive concentration level. From these results it will be inferred that (1) concentration in the prostate gland or prostatic cancer tissue must be smaller than expected, (2) there is no probability of prostatic scintigram by 131I labeled diethylstilbestrol diphosphate, and (3) after dephosphorylation, most of stilbestrol passes almost immediately into general circulation and the passage, if it is transient, takes cytostatic effect for prostatic cancer.