Enhanced production of prostaglandins (PGs) by experimentally-induced and naturally occurring tumors and their effect on tumor growth and immunosurveillance have been noted. Directed toward further evaluation of the relationship between prostatic tumor growth and its milieu, i.e., microenvironment, we investigated the possible correlation between levels of PGs, tumor size, and metastatic potential. For this purpose, the levels of PGE2 and PGF2 alpha in plasma and tumor effusions of three tumor sublines of the Dunning R-3327 rat prostate adenocarcinoma were measured: R-3327H, well-differentiated, slow-growing, and poorly metastatic; R-3327G, poorly differentiated, fast-growing, and poorly metastatic; and R-3327 Mat LyLu, anaplastic, fast-growing, and highly metastatic. The level of PGF2 alpha was highly variable with no significant differences being noted between the tumor sublines. The mean values of PGF2 alpha were, however, higher, although not significantly so, in the smaller tumors within each of the sublines. The levels of PGE2 were significantly higher in Mat LyLu effusions than those from the nonmetastasizing R-3327G and H sublines. Evaluation and comparison of the relationship between tumor burden, i.e., size versus levels of PGE2 and PGF2 alpha showed no significant differences. A vasodilator and regulator of immunological responsiveness, PGE2, may function as a modulator of tumor metastases. In consonance with studies by others elevated levels of PGE2 may possibly serve as a prognostic marker for the high metastatic potential of neoplastic cells.