In two experiments with rats as subjects, the temporal characteristics of inhibition produced through extinction were investigated. Each experiment established two independent signals for unconditioned stimulus presentation, one trace and one delay. Following initial training, either the trace or the delay conditioned stimulus (CS) was massively extinguished. In Experiment 1, a summation test established that an extinguished delay CS (but not a neutral CS) passed a summation test with a delay, but not with a trace, transfer excitor, and an extinguished trace CS (but not a neutral CS) passed a summation test with a trace, but not with a delay, transfer excitor. In Experiment 2, a retardation test showed retarded behavioral control by an extinguished delay CS when the CS was retrained as a delay CS, but not as a trace CS, and by an extinguished trace CS when the CS was retrained as a trace CS, but not as a delay CS. The results are discussed in terms of contemporary theories of extinction.