Used a behavioral tracking technique to investigate dark adaptation in 3 American red squirrels. Dark adaptation was fully completed within 30 min after the offset of the longest light-adaptation duration. An increase in visual sensitivity of more than 3 log units had taken place, and the final threshold (10-5 ml) was about 1 log unit above the scotopic threshold of a human tested in the same apparatus. A discontinuity occurred about 4 min after the start of dark adaptation when the Ss were light adapted for 4 min. No rod-cone break occurred when the Ss were not light adapted above room illumination. When colored test stimuli (red, yellow, and blue) were used after 4 min of light adaptation, rod-cone breaks occurred for each test stimulus. It is concluded that the tree squirrel possesses a functional duplex retina. (19 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)