In a previous study the frequency of successful imprinting in chicks was found to increase with age to a maximum of approximately 50% at the age of 13-16 hours after hatching, and to decrease thereafter. The 1st study reported here shows that in white rock chicks the frequency of fear responses elicited by the potential imprinting object starts to increase at the age of 13-16 hours, which could account for the decrease in frequency of successful imprinting beyond the age of 15-16 hours. The 2nd study demonstrates an increase in locomotor ability in white rock chicks as a function of age with near asymptotic performance being reached 13-16 hours after hatching, which could account for the increase in frequency of successful imprinting up to the maximum of 13-16 hours. From Psyc Abstracts 36:02:2EC15H. (PsycINFO Database Record (c) 2006 APA, all rights reserved)