the progressive ratio (pr) schedule of reinforcement is widely used to assess reinforcement strength. rats typically complete higher ratio requirements for more palatable food. this effect is well characterized by the mathematical principles of reinforcement (mpr). in this experiment, either standard food pellets or amaranth grain were delivered according to pr schedules, with requirements increasing in steps of 1 (pr 1) or 3 (pr 3). rats completed higher ratio requirements and displayed shorter pre-ratio pauses for pellets than for amaranth. nevertheless, run-rates were similar across reinforcers. in terms of the theoretical parameters of mpr, similar values in a parameter (which indices incentive value) were obtained for amaranth and pellet reinforcers. this finding suggests that, although amaranth engenders less responding than pellets, amaranth grain is a reliable reinforcer for operant procedures. the mpr model accounted for the differences in quality of food.