The effects on conditioning performance and extinction of a Fixed Effort FE schedule of reinforcement were investigated. A FE schedule is programmed by requiring that the cumulative sum of the efforts of individual responses be larger than a pre-selected constant criterion specified in gram-seconds. The effort of a response is defined as the time integral of force of the response. The major findings reported in this study are a decrease in reinforcement rate as a function of the increased effort requirement per reinforcement an apparent inhibiting effect of high effort requirements on effort per response no effect of effort per reinforcement on resistance to extinction a significant increase in the variability of effort per response during extinction a significant difference between the effort distributions of the 60 gm. -sec. differentiation animals and the 60 gm. -sec. FE animals and a correlation between the direction and amount of change of effort per response from conditioning to extinction.