Background: Cytokines play a central role in inflammatory responses and in specific immune responses directed toward alloantigens. The pattern and quantity of cytokines produced in graft rejection can yield valuable information regarding the cellular and molecular mechanisms of the antigraft response. Methods: We used the polymerase chain reaction to semiquantitatively measure changes in the amount of messenger RNA from the interleukin-1β, interleukin-2, interleukin-4, interleukin-6, interleukin-10, tumor necrosis factor-α, interferon-γ, interleukin-1 receptor antagonist, and the interleukin-2 receptor genes in the peripheral blood and endomyocardium of cardiac allograft recipients during the first 8 weeks after transplantation. A total of 328 samples of resting (n = 251) and stimulated (n = 77, stimulated with phytohemagglutinin and lipopolysaccharide for 18 hours) peripheral blood mononuclear cells collected from 16 patients were measured. To measure intragraft cytokine levels, we analyzed 150 endomyocardial biopsy specimens from 19 patients. Results: No elevation in expression was seen before rejection, but, after the onset of rejection and concomitant with treatment, there was a decrease in detectable mRNA (p < 0.05) for the pro-inflammatory monokines interleukin-1β, interleukin-6, and tumor necrosis factor-α in resting peripheral blood mononuclear cells, and a decrease for the T-cell derived cytokines interleukin-4 and interleukin-10 in stimulated peripheral blood mononuclear cells. These changes in mRNA expression occurred coincidentally with decreases in the percentage of lymphocytes and monocytes in the peripheral blood after administration of rejection therapy. In endomyocardial biopsy specimens, there were no detectable changes in the quantities of cytokine mRNA species for the interferon-γ, interleukin-6, interleukin-10, interleukin-1ra, and interleukin-1β genes before rejection. In general, the levels of these cytokines were near the lower limits of detection by our assay in endomyocardial biopsies. mRNA from the interleukin-2, interleukin-4, tumor necrosis factor-α, and interleukin-2R genes were undetectable. Conclusions: We conclude that changes in the expression of cytokine mRNA in both peripheral blood mononuclear cells and endomyocardial biopsy specimens as measured by the semiquantitative polymerase chain reaction method used in this study does not effectively predict rejection. The decline in peripheral blood mononuclear cell cytokine mRNA after rejection treatment is likely due to changes in the proportion of lymphocytes and monocytes in the peripheral blood in concert with a steroid-induced downregulation of cytokine gene transcription.