Microscopic evidence of inflammation was observed in the synovium of rabbit knees that had been injected with 10 ppm of Cidex (2% glutaraldehyde). Initial changes were synovial hypertrophy, subsynovial edema, and vascular congestion. At concentrations of 100 ppm or greater, focal synovial necrosis, hemorrhage, and gross diffuse synovitis were observed. The degree of synovial inflammation was proportional to the concentration of Cidex. Observation of synovial response versus time demonstrated this reaction to be a chemical synovitis without histologic evidence of a delayed hypersensitivity allergic component. Significant intraoperative levels of Cidex were noted in the rise solutions at several hospitals. When a single rinse is used, the concentration of Cidex present in the rinse basin is 100 to 300 ppm. If the same rinse is used for subsequent cases, the Cidex concentration is on the order of 1,000 ppm by the fifth arthroscopic procedure. If a double rinse is used and the rinse changed with each operative case, the Cidex concentration in the second rinse is less than 10 ppm. After irrigation of the knee joint with 1 liter of saline, the intraarticular concentration of Cidex is less than 10 ppm regardless of the rinse technique.