The VO2 response to a step change in work rate resembles a first order exponential function. This suggests that a single site of the many which link ATP demand to VO2 is limiting. The location of this site is controversial. Some authors suggest that O2 delivery regulates the VO2 response. Others have suggested that peripheral mechanisms within the active muscle regulate O2 consumption. The evidence supporting each hypothesis is reviewed. It is concluded that O2 delivery sets the initial parameters and peripheral mechanisms then regulate O2 utilization within the bounds initially established by delivery mechanisms.