1. The effects of intracerebroventricular administration of Tyr-d-Arg-Phe-β-Ala-NH 2 (TAPA), a novel dermorphin analog, on plus-maze learning and spontaneous alternation performance were investigated in mice.2. The pre- or posttraining or preretention administration of TAPA (0.3-3.0 ng) alone failed to affect transfer latency of plus-maze learning, whereas TAPA (3 ng) produced a significant decrease in percent alternation without affecting total arm entries.3. β-Funaltrexamine (5 μg) almost completely reversed the TAPA (3 ng)-induced decrease in percentage of alternation.4. These results suggest that stimulation of μ-opioid receptors disrupts spontaneous alternation performance associated with spatial working memory.