In a hypothesis derived largely from animal literature, Douglas proposed that the hippocampus in the human brain matures at about 4 to 5 years of age. In the present experiment, this was investigated with maze learning in preschool children. As predicted, younger preschoolers exhibited deficits observed in hippocampectomized adult animals, namely, perseveration, inflexibility, and invariability, in terms of several measures of errors and routes selected. Older preschoolers exhibited the flexibility and variability more characteristic of an adult animal with intact hippocampus. Thus, the base of support for the hypothesis was broadened beyond spontaneous alternation data reported by Douglas. (PsycINFO Database Record (c) 2006 APA, all rights reserved).