Effects of age on olfactory discrimination learning involving odorants varying in perceptual similarity using a longitudinal design. Graphs show the percentage of correct trials (in 20-trial blocks) on 3 odor discrimination problems in which the odors (selected from a series of aliphatic alcohols) varied in the difference between the numbers of carbon atoms and hence their perceptual similarity. Δ5, Δ3, and Δ1 refer to differences of 5, 3, and 1 carbon atoms between the 2 odorants in each problem. Rats were tested on the same discrimination problems at 3 different time points. Performance was worse on problems with more perceptually similar stimuli but improved across time points with repeated testing. Abbreviation: SEM, standard error of the mean.