In vitro formation of prostaglandins (PG) E₂, F_2α, 6-keto-F_1α, and thromboxane B₂ (TxB₂) by glomeruli from rats with reduced renal mass (RRM) were evaluated by radioimmunoassay. Four weeks following ablation of renal mass, PGE₂, PGF_2α, and TxB₂ production by glomeruli from RRM rats was significantly greater, when compared with glomerular PG and TxB₂ production of sham-operated control (C) rats. The effect of cyclooxygenase inhibition with indomethacin and the selective inhibition of thromboxane formation with UK 38485 on glomerular filtration rate (GFR) was investigated in experiments in vivo. In RRM rats indomethacin reduced GFR from 212 ± 17 to 138 ± 14 μl/100 g/body weight (p < 0.05) without effect on C rats. Thromboxane synthesis inhibition with UK 38485, however, increased GFR significantly in RRM rats (221 ± 26 to 303 ± 21; p < 0.05). The data suggest that vasodilatory PGs and TxB₂ modulate GFR in rats with ablation of renal mass.