The effect of force requirements on response effort was examined using inbred C57BL/6J mice trained to press a disk with their snout. Lateral peak forces greater than 2 g were defined as responses (i.e., all responses above the measurement threshold). Different, higher force requirements were used to define criterion responses (a subclass of all responses) that exceeded the requirement and produced a reinforcer. The reinforcer was sweetened, condensed milk, delivered upon response termination. All mice were exposed to two ascending series of criterion force requirements (2, 4, 8, 16, and 32 g). Increasing the force requirement initially decreased criterion response rates, but criterion response rates recovered with continued exposure, except at the 32-g requirement. Response rates for all measured responses initially increased with increasing force requirements, but then decreased with continued exposure. The second exposure series produced more stable response rate changes than the first series. The time-integral of force (area under the force-time curve for individual responses, which is proportional to energy expenditure for each response) increased with the increase in the force requirement. The C57BL/6J inbred strain generated average force output similar to CD-1 outbred stock mice trained on the same force requirements. C57BL/6J inbred strain mice differed from CD-1 mice in initial response rates (for all responses above threshold) and had lower response rates at the 16 and 32 g requirements resulting in lower total force output. These data show for both mice types that increased force requirements resulted in increased overall responding (all measured responses), which contradicts a punishment interpretation of criterion response decrements. C57BL/6 inbred mice showed individual differences comparable to the outbred CD-1 stock. C57BL/6 mice did not maintain responding as well at the higher force requirements, which may be due to their small body size and weight, compared to the larger and heavier CD-1 mice. (Contains 1 table and 10 figures.)