Examined the effects of melanocyte-stimulating hormone (MSH) on attention and memory under conditions of constant darkness and constant illumination in a study with 60 male Holtzman albino rats. After acquiring a brightness discrimination problem, Ss were given either a reversal or memory task. Ss treated with MSH and those tested in the dark performed the reversal task significantly faster than Ss tested under constant illumination. Although differences in memory were detected between groups, these were not statistically significant. Results are interpreted to suggest that increased MSH levels lead to increased awareness of the environment in the rat and that functionally equivalent behavior among other species may derive from similar neuroendocrine substrates. (PsycINFO Database Record (c) 2006 APA, all rights reserved)