Pigeons were exposed to a two-component multiple fixed-ratio X fixed-ratio Y schedule of reinforcement in which X was always less than Y. Components were equal in duration and alternated at rates that varied between 2 sec and 23.6 h. Relative response rate in the FR X component: (1) increased as the duration of components increased between 2 sec and 16 min, (2) was at a maximum between 16 min and 6 h, and (3) decreased as the duration of components increased from 6 h to 23.6 h. The changes in relative response rate were attributable primarily to changes in absolute response rates during the FR Y schedule as absolute response rates during the FR X schedule were relatively invariant. These results pose complexities for several theoretical formulations.In a multiple (mult) schedule, two or more independent reinforcement schedules are alternated and each of the component schedules is accompanied by a different stimulus. Although the four most common simple schedules, fixed interval (FI), fixed ratio (FR), variable interval (VI), and variable ratio (VR), have all been studied as components of mult schedules, VI schedules have received the most attention. The comparison of different schedule combinations, although important in its own right, is also relevant to several attempts to integrate data from a wide range of experimental procedures (Catania, 1973;Herrnstein, 1970Herrnstein, , 1979Rachlin, 1973;Shimp, 1969Shimp, , 1973. Most of the data cited by these investigators, in support of their respective positions, come from experiments employing interval, rather than ratio, schedules.Data from ratio schedules would appear to permit an assessment of the power and generality of these formulations. It is easy to see, however, why data from mult or concurrent (cone) ratio schedules are seldom included in these analyses. In the first place, the few studies that have examined responding on mu/t FR FR schedules may not be comparable to those involving mu/t VI VI schedules. For example, the transition from one component to the next in a mu/t VI VI has been response independent (Lander & Irwin, 1968;Nevin & Shettleworth, 1966;Reynolds, 1963;Shimp & Wheatley, 1971;Todorov, 1972); that is, the components of a two-component mu/t schedule alternate at some specified interval, independently of the animal's behavior. With mult 51FR FR, however, the transition has been response dependent, since the components typically alternate after some specified number of reinforcements (Crossman, 1968(Crossman, , 1971Findley, 1962; Schuster, 19S9). More importantly, the relation between responding and reinforcement rate, of central interest on interval schedules, is not free to vary on ratio schedules. However, experiments by Shimp and Wheatley (1971) and Todorov (1972) with mu/t VI VI schedules suggest a promising procedure for studying behavior maintained by mult FR FR reinforcement schedules and for meaningfully comparing such behavior with that reported in the mult VI VI case. These studies examined the rates of responding on a mu/t VI VI ...