With interest in steroidogenic acute regulatory protein (StAR) involvement in the luteolytic process, we studied changes in serum progesterone levels and the concomitant expression of StAR mRNA and protein (37-, 32-, and 30-kDa forms) in postovulatory Day 7 corpora lutea (CL) isolated from rats 1 h after injection with prostaglandin F_2α (PGF_2α, n = 6) or saline (n = 6). Serum progesterone levels were determined by RIA, StAR and β-actin mRNA expression by Northern analysis, and StAR and β-actin protein expression by Western analysis. Adrenal, brain, and spleen from control animals were used as positive and negative controls for StAR expression. Scanning optical densitometry measurements were standardized by dividing the signal strength from each StAR autoradiogram lane by that from the corresponding β-actin autoradiogram lane. ANOVA was used for significance testing, with α set at 0.05. The 37-, 32-, and 30-kDa forms of StAR protein were expressed in all adrenal samples, whereas only the 37- and 30-kDa forms were found in CL. Serum progesterone levels and expression of the 30-kDa and 37-kDa forms of the StAR protein in CL were all found to be significantly lower in the PGF_2α-treated than the saline-treated group. StAR mRNA expression was not significantly different in the saline- and PGF_2α-treated rats. The rapid decline in StAR protein expression that accompanies PGF_2α induced luteolysis, therefore, does not result from significant decline in mRNA expression.