The effects of pre and post-passive avoidance training, and exposure to different wavelengths of light on numerical synapse density and the mean projected synapse height in the intermediate and medial part of the hyperstriatum ventrale (IMHV) of the chick brain were investigated. Synaptic counts and the mean projected synapse height measurements were done using a 3-dimensional stereological probe, the disector, which requires no assumptions about size, shape, and orientation of synapses. It was found that there was a significant increase in the density of synapses in the left IMHV of chicks in white, yellow and red light illuminations respectively. However, the mean projected height of synapses in these chicks was significantly increased in the right IMHV under white, yellow, green but not red light illumination conditions. These results suggest that numerical and structural modification of synapses may depend on wavelengths of white, yellow, red and green lights. © 2010 OMU All rights reserved.