The pecking response of pigeons was reinforced when directed at a key transilluminated by chromatic light. Subjects were assigned to one of three sequences of S+ and S- stimuli and to one of two multiple schedules of reinforcement. The AE Group (acquisition - extinction) received daily a block of nine S+ periods followed immediately by a block of nine S- periods. The EA Group received the reverse order of stimuli while a control group (R Group) received a random sequence of stimuli. Each of the ' above sequences of stimulus presentation was in effect in conjunction with either a Mult VI-1 min - Ext schedule or a Mult VI-1 min - VI-4 min schedule of reinforcement. A total of six groups were thus obtained