Ketamine administration following reinstatement decreases fear following three-shock CFC, but not one-shock CFC. (a) Experimental design. (b) Mice injected with saline and ketamine 1 h after one-shock reinstatement express comparable levels of fear behavior during secondary extinction trials (n=5 mice per group). (c) Experimental design. (d) Mice injected with ketamine 1 h after three-shock reinstatement express decreased fear when compared with mice injected with saline (n=9–10 mice per group). (e) Mice injected with saline or ketamine 24 h after three-shock reinstatement express comparable levels of freezing behavior during secondary extinction trials (n=5 mice per group). Error bars represent±SEM. *p<0.05, and **p<0.01. CFC, contextual fear conditioning; E, extinction; K, ketamine; R, reinstatement; Sal, saline.