In three separate experiments the timing abilities of brush tail possums and domestic hens on the peak procedure was investigated. This procedure involved animals responding on two trial types within an experimental session. On some trials responding was reinforced according to a Fixed Interval (FI) schedule (in effect on 80% of trials), and on other 20% trials, Peak Interval (PI) trials, responding was not reinforced with food. Possums lever pressed, and hens key pecked, for food reinforcers on different FI schedules, and the duration of the PI was varied across a range. Response rates typically increased to a maximum at about the time the responses were normally reinforced and then decreased after the time that food would normally be reinforced, before increasing again towards the end of the PI regardless of the duration of the PI trial if that duration was fixed. When the PI was of variable rather than fixed duration, however, the rate of responding on PI trials decreased towards the end of the PI. When relative response rates were plotted as a function of relative time the function typically superposed for the ascending, but not descending portions of the function. The results are discussed in terms of Webers law, and various quantitative models timing.