Mathematical Principles of Reinforcement (MPR, Killeen, 1994) is a mathematical model comprising three main concepts; an animal’s arousal to behaviour based on its motivation for a particular reinforcer, time and energy constraints on responding, and coupling between a response class and reinforcer. This experiment tested the ability of MPR to predict response rates when the minimum force requirement and topography of response was changed. Increasing the minimum force requirement was expected to increase the value of δ, the parameter related to response constraint. Altering the topography of the response was expected to also alter the δ value, as different response forms were expected to take different lengths of time to perform. There were four conditions; low force key, low force door, high force key and high force door, and 6 hens responded under each of these conditions in an ascending geometric series of Fixed Ratio (FR) values. It was shown that hens responded at a faster rate and to higher FR values when responding on the key than on the door, and for both apparatus, the hens stopped responding at lower FR values when weights were added. Unexpectedly, there were no statistically significant differences in the value of δ across conditions, but the values for a, meant to represent the animals’ arousal, did change. It was suggested that the changes in a reflected changes in the animals’ motivation to perform the different responses, probably due to rewarding or aversive properties of the operant response related to the different response forms.