Systemic dexamethasone, cyclosporine A, colchicine, indomethacin, and cobra venom factor were administered to Lewis rats that were then challenged with 100 micrograms of Salmonella typhimurium endotoxin. The effects of these five pharmacologic agents in endotoxin-induced uveitis were studied by analyzing their effects on ocular inflammation and on the expression of class II antigen of the major histocompatibility complex (MHC) on the epithelium of the ciliary body and iris. All the agents used in our study exhibited anti-inflammatory effects but only dexamethasone was able to inhibit significantly the expression of Ia antigen on the uveal epithelium. This demonstrates a dissociation between the inflammation and class II antigen expression and supports our previous data that class II antigen presentation may precede the inflammatory infiltrates. The alteration of ocular inflammation and especially the variation of Ia antigen expression in the uveal tissue may provide insight into the mechanism of action and the success of different pharmacologic agents used in the treatment of uveitis.