(A) Freezing during fear conditioning before (pre-CS), during (CS1 and CS2) and after (post-CS) the two conditioning trials in wildtype (WT), heterozygote (D1 HET), and knockout (D1 KO) mice. (B) Average freezing to the context during acquisition (Acq CTX A) and during an extinction test 24 hr later (Ext CTX A), and freezing to the CS during daily cued extinction sessions in Context B (Tests 1–6 CTX B), and during a test in the acquisition context (Renewal CTX A). (C) Freezing during CS-off periods during the cued extinction tests. D1 KO mice showed increased freezing in response to the CS on Test 1 (compared to WT) and on Test 2 (compared to WT). D1 KO mice showed a trend towards increased freezing during CS-off periods on Test 1. D1 KO mice showed a non-significant decrease in freezing during CS presentations compared to WT during Renewal. Error bars indicate SEM. (*) p < 0.05 significant difference compared to WT. (°) p = non-significant trend for KO compared to WT.