Experimental design: Gonad-intact, aged rhesus females were trained to criteria during task acquisition. All but 6 of the animals were then bilaterally ovariectomized, and all 28 were retested after 1 week, 1 month, and 2 months during the acute OVX testing phase. Longitudinal hormone replacement began thereafter, and all groups were retested at 1 week and at 1, 2, 4, 6, 9, and 12 months.