Rats have been used to study the function and development of the mammalian prostate. Identification of prostatic secreted proteins is important in order to better understand their physiological function. Previous investigations have showed that prostatein, cysteine-related protein 1, and kallikrein S3 are in the ventral prostate (VP), whereas the proteins probasin, prostate secretory peptide 94, transglutaminase 4, and carbonic anhydrase II are produced in the lateral prostate, dorsal prostate (DP), and anterior prostate. They are also useful markers when looking at androgen dependency as well as prostate-specific expression. Although some of the rat prostatic proteins have been investigated well, the overall protein expression profile of the prostate has not been examined. In the present study, the secretions from the rat prostate were subjected to 2-dimensional gel electrophoresis followed by mass spectrometric analysis. In addition to the previously known proteins, proteome analysis revealed several new secreted proteins, including spermine-binding protein and a protein similar to immunoglobulin-binding protein. In addition, epididymal secreted protein 1 and peroxiredoxin 6 were found in the DP, while glucose-regulated protein 78 was identified in all lobes of the prostate. Castration of the animals led to a decrease in the mRNAs of all of these secreted proteins. While the mRNAs of prostatic proteins became almost completely absent in the VP, the reductions in the other lobes were limited. A novel view of rat prostate secretion from our results should contribute to an understanding of the biological functions of the prostate gland.