This study determined the effects of estrogen on attention and motor speed in young monkeys (Macaca fascicularis). Performance on visuospatial attention and reaction time motor tasks was measured before ovariectomy, for 2 months after ovariectomy, and at 14 months after treatment with placebo or estrogen replacement therapy (ERT). Release time on invalid trials in the attention task was increased after ovariectomy and was reduced in monkeys treated with ERT. but not placebo. Simple reaction time was not altered after ovariectomy or treatment. The effects of scopolamine on attention, but not memory, in a delayed response task were dependent on estrogen. These observations indicate that loss of estrogen modulates visuospatial attention in primates and that 1 mechanism of this modulation is through the cholinergic system.