Male CD1 mice received 20 pairings of tone and footshock (FS) or tone alone in an arm of a Y-maze on Day 1. On Day 2 either extinction (tone alone) or no extinction was followed immediately by saline or picrotoxin (0.5 or 1.0 mg/kg ip). Nonextinguished groups received only saline or picrotoxin (1.0 mg/kg ip) on Day 2. Other groups received saline or picrotoxin (1.0 mg/kg) 2 hr after extinction. On Day 3 all mice were placed in the Y-maze (with doors to all 3 alleys open), and total alley entries during a 2-min test session were recorded. Day 1 FS training resulted in reduced alley entries during the test session. Day 2 extinction session significantly attenuated the effects of the FS training. Day 3 performance of mice given picrotoxin (1.0 but not 0.5 mg/kg) immediately postextinction was comparable to that of mice not given FS on Day 1. The findings suggest that picrotoxin enhanced extinction of conditioned fear.