Compounds of Formula I: I (where variables R1A, R1B, R2, R3, R4, A, and Z are as defined herein) useful as antagonists of CGRP receptors, and useful in the treatment or prevention of diseases in which CGRP receptors are involved, such as headache, and in particular migraine and cluster headache. The invention is also directed to pharmaceutical compositions comprising these compounds and the use of these compounds and compositions in the prevention or treatment of such diseases in which CGRP receptors are involved.