Circadian variations in alpha-methyl-p-tyrosine's (alpha MT) disruption of active avoidance acquisition were studied, alpha MT administered between 11.0 and 13.00 h, 17.00 and 19.00 h and 23.00 and 01.00 h impaired learning, but alpha MT administered between 05.00 and 07.00 h did not impair learning. The time of alpha MT ineffectiveness corresponds with the reported circadian peak in brain catecholamines, suggesting that the circadian susceptibility rhythm of alpha MT depends upon circadian fluctuations of brain catecholamines.