Yohimbine leads to extinction, whereas propranolol leads to incubation, of cue fear when given in conjunction with spaced CS presentations. Separate experiments examined the effect of vehicle, yohimbine, or propranolol on the effect of spaced CS presentations after weak (A,B; see Materials and Methods) or strong (C,D; see Materials and Methods) cue fear conditioning (eight mice/group). In each experiment, 1 d after acquisition, mice were injected with vehicle, yohimbine, or propranolol and presented with seven CSs (20 min ITI). (A,C) Freezing during the spaced CS sessions 20 min after injections. (B,D) Freezing 1 d later during the final 5-min CS tests (drug free). (*) p < 0.05 versus RC mice; (+) p < 0.05 versus vehicle-exposure mice.