Egocentric learning ability of developing serotonin (5-HT)-depleted female rats was evaluated in the Morris maze test. 5-HT depletion was accomplished by a unique intracisternal injection of 5,7-dihydroxytriptamine at 21 days-old. A first behavioral test was applied before the lesion procedure. The animals were thereafter challenged to resolve the same test at 40 and 60 days-old. 5-HT depletion caused a dual effect on the rats' egocentric learning ability, i.e. at 40 days; control rats learned the task while the experimental rats were unable to learn it. At 60 days, control animals were unable to learn the test, while the experimental rats showed a successful performance. These results strongly suggest that 5-HT neurotransmission is necessary for egocentric learning establishment and regulation.