Cytochrome oxidase activity was examined in the striate cortex (area 17) of squirrel monkeys at both the light and ultrastructural levels. Two prominent bands of reactivity were found in 4A and 4C with intermittent puffs of cytochrome oxidase reactivity in laminae 2 and 3. These puffs, spaced 0.5 mm apart, were in register with intermittent concentrations of activity in laminae 4B, 5, and 6. A thin band of reactivity was observed in lamina 1. The upper portion of 4C beta was less reactive than 4C alpha or the lower portion of 4C beta. Reactive neurons included stellate cells in all laminae and pyramidal cells in laminae 2 through 4B, 5, and 6. A row of large reactive pyramidal cells was observed in upper lamina 6. More reactive neurons were found in the puffs (laminae 2 and 3) than were observed in interpuff regions, and the reactive neurons were significantly larger than the nonreactive neurons. Reactive neurons contained two to three times as many reactive mitochondria as did the nonreactive neurons and often had indented nuclei. Based on the number of darkly or highly reactive, moderately reactive and lightly reactive mitochondria, puff regions were significantly different from nonpuff regions; there were approximately two times as many darkly reactive mitochondria in puff regions as compared to a similar nonpuff area. The majority of mitochondria (32% in puff; 44% in nonpuff) were found to reside in the dendritic profiles, which also contained the majority of highly reactive mitochondria. In a separate analysis, the total area of highly reactive mitochondria within puff regions was found to be twice the total area of highly reactive mitochondria in a comparable nonpuff region. An analysis of synapses showed that there were more asymmetrical synapses in both puff and nonpuff regions (55% and 54%, respectively) than symmetrical ones (45% in puff and 46% in nonpuff). There was an increase in mitochondrial reactivity in both asymmetrical and symmetrical synapses in the puff areas; however, the increased reactivity within asymmetrical terminals was significantly greater than that within symmetrical ones. Several somatodendritic synapses were observed and they were all of the symmetrical variety. Axospinous contacts were primarily of the asymmetrical type; however, symmetrical axospinous synapses were observed and were typically seen in association with an asymmetrical synapse. It was concluded that cytochrome oxidase activity is localized primarily within the dendritic profiles in both puff and nonpuff regions.(ABSTRACT TRUNCATED AT 400 WORDS)