The recently discovered endogenous mu-selective opioid peptide, endomorphin-2, and the endogenous delta-selective opioid peptide, Leu-enkephalin, were tested for their ability to affect spatial learning in the Morris water task. It was found that microinjection of 10 nmol endomorphin-2 into the CA3 region of the rat hippocampus significantly impaired spatial learning. However, the two lower doses tested, 3.3 and 1 nmol, had no effect in this test. Leu-enkephalin did not have any effect on spatial learning at the two doses tested (10 and 3.3 nmol). Neither peptide had any effect on motor performance as measured by swim speed. The results indicate that mu-receptors in the CA3 region of the rat hippocampus are more relevant than delta-receptors for spatial learning.