The aim of this study was to characterize the synthesis and metabolism of platelet activating factor (PAF, 1-0-alkyl-2-0 acetyl-sn3-phosphorylcholine) by renal tissue undergoing acute cellular allograft rejection in the canine model. Kidneys were transplanted into outbred mongrel dogs and allowed to reject without immunosuppressive therapy. Five days after transplantation, all kidneys were non-functional and the tissue was assayed for the capacity to produce various molecular species of PAF and lyso-PAF using physical-chemical (GC/MS), immunologic (RIA) and biologic (platelet aggregation) assays. Renal cortical tissue obtained from rejecting allografts produced more PAF than control tissue by the following factors (GC/MS): 18-fold for C16:0 PAF; 3-fold for Lyso-C16:0 PAF; 2-fold for C18:1 PAF; and 6-fold for C18:0 PAF. The control tissue to which comparisons were made was renal cortex obtained from the native contralateral kidney. Increases in the production of various molecular species of PAF were also observed with renal medullary tissue undergoing acute rejection, although the magnitude of change was less dramatic than with renal cortex. The predominant PAF metabolite produced both by normal and allograft tissue was C16:0 Lyso-PAF. The increased PAF production by renal allograft tissue undergoing rejection was mainly attributable to C16:0 PAF and C16:0 Lyso-PAF, but increased production of both C18:0-PAF and of C18: 1-PAF was also detected. Increased renal allograft PAF production was also confirmed with a competitive binding immunoassay specific for PAF. In addition, when PAF-like material was isolated and purified from renal allograft incubation media and added to washed canine platelets, an intense aggregation response was observed that was abolished with prior alkaline methanolysis of the isolated material. Aggregation responses of similar magnitude were not obtained with PAF-like material isolated from native (non-rejection) renal tissue. In other experiment, incubation media obtained from rejecting renal allografts was found to contain factor which catalyzed hydrolysis of exogenous PAF to Lyso- PAF at twice the rate induced by media obtained from normal renal tissue. In conclusion, this study has identified dramatic increases in production of the biologically active molecular species of PAF by renal allograft tissue undergoing untreated cellular rejection. High levels of biologically inactive Lyso-PAF were also detected, and renal allograft tissue elaborates a factor which catalyzes rapid hydrolysis of PAF.