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Conditioned blocking (CB) is the undermining of conditioning to a stimulus by conjoint exposure with one already associated with the unconditioned stimulus. CB is one of several tests of “learned inattention” in which performance has been found to depend on personality features of human participants and monoamine activity in animals. In Part 1, the performance of 25 healthy young adults on a new test form for demonstrating CB is described. From personality inventories and 24-hr urine samples it was proposed that CB may be correlated with extroversion and increased catecholamine utilization. In Part 2, CB was shown to be present in 4 groups of 11 participants with mean ages of 10, 14, 17 and 22 years independent of IQ, but it was least marked in the prepubertal group. No relation of performance with personality features was found. As with the adult group, CB was positively correlated with dopamine activity, but unlike the adults it was negatively correlated with noradrenalin activity. The maturation of attention-related information processing is discussed in terms of the development of limbic structures and dopaminergic versus noradrenergic function.