Four pigeons and four rats were exposed to a Fixed-interval 60-sec reinforcementschedule, and each of four different durations of an extemal clock were added to eachof four different parts of a session. For the birds the stimulus dock was a color in thepecking key and for the rats it was a tone. Results showed that this procedure affectedsolely the birds' behavior. Control condition replicated Lowe and Har%em 's (1977)finding of interspecies differences of responding under fixed-interval schedules. Thepresent results are explained appealing to interacuons between temporal and situationalcontrol, and to differential sensitivity of operants to these types of control.