Differential reinforcement of low rate responding (DRL) can be defined as withholding reinforcement until probability of a target behaviour has decreased to a particular criterion. Typically, the DRL lowers the overall rate of a specific target response by incrementally increasing the time interval between responses and delivering reinforcement contingent upon a response occurring at or above the specified time interval. The present study parametrically examined six brush tail possum’s performance on DRL to investigate in the first instance whether or not the possums could perform on DRL and if so, how well they could do at the task. The study also sought to investigate if DRL performance could be improved, by providing a second response lever as an explicit option to perform an alternative behaviour. Two Experiments were carried out. In Experiment 1, possums were expected to perform on a DRL schedule with one response lever (DRL lever) that produced access to food. In Experiment 2, a second response lever was available independent to the DRL lever and accessible for the duration of the experiment. The second lever provided no access to food. The possums were expected to respond on the second lever during the time delays (inter response time (IRT)) to assist mediating the time interval. Initial inspections of the data in Experiment 1 and Experiment 2 showed the possums performed efficiently on the DRL schedule up to around 15-s. Closer inspection of mean IRT data showed more unsuccessful responses than successful responses at around DRL 10-s and higher. Most of the possums maintained consistent responding below the DRL but sustained enough responding above the DRL to go on to higher DRL criterions. In spite of the larger number of unsuccessful responses some of the possums still failed to reach higher DRL criterions. Further inspection of the procedure revealed that at longer DRL as long as the possums had enough time (in days) to obtain the required number of reinforcers to meet the DRL, the procedure would automatically increment to the next DRL criterion giving the impression that they were able to perform the DRL. Furthermore, excessively long IRT were reinforced in the procedure. The provision of a second response lever in Experiment 2 was of no consequence contrary to expectations. This thesis purports that possums did not respond to the DRL contingencies and that performance that appears as if they did respond was an accidental by-product of the procedural arrangement.