Carbon monoxide (CO) produced by internal combustion engines is an indoor health hazard. High CO levels can occur during indoor sporting events--such as tractor pulls--that involve vehicles modified to achieve high horsepower. In January and March 1992 and January 1993, the Cincinnati Health Department evaluated CO levels during tractor pulls, monster-truck jumps, and a mud race event held in an indoor arena with a seating capacity of approximately 16,000 persons. This report summarizes findings from the evaluations.