WHEN young zebra finch males who have been reared in isolation court a female for the first time they develop a stable preference for females of this species (sexual imprinting). During this first courtship, two areas of the forebrain show a reversible enhancement of spine density, while in two other areas, spine density decreases irreversibly. Here we show that the same alterations can be induced by application of tetraethylammonium (TEA) to slices of adult, previously isolated males. TEA application induces an enhancement of spine density in two forebrain areas and leads to a decrease of spine density in two others. Although the exact mechanisms are unknown, our results indicate that potentiation phenomena are involved in the spine density changes induced by first courtship.