The effects of several calcium antagonists, including nicardipine, nifedipine, verapamil, and diltiazem, on muscarinic acetylcholine antagonist binding were studied in the P2 fraction of the rat cerebral cortex using either [3H]quinuclidinyl benzilate ([3H]QNB) or [3H]pirenzipine as the radioactive ligand. (1) The potency of [3H]QNB binding inhibition was in the order nicardipine greater than verapamil greater than diltiazem greater than nifedipine. The IC50 values of nicardipine, verapamil, and diltiazem were 2.56 X 10(-6) M, 1.28 X 10(-5) M, and 6.00 X 10(-5) M, respectively. (2) The inhibition of [3H]QNB binding by nicardipine was significantly decreased in the presence of Ca ions. (3) In saturation experiments of [3H]QNB binding in the presence of nicardipine, the Kd value appeared to be significantly affected, but the Bmax value was unchanged. This indicated that nicardipine probably inhibited [3H]QNB binding allosterically. On the other hand, (4) nicardipine inhibited [3H]pirenzipine binding completely with an IC50 value of 7.87 X 10(-7) M. It was concluded that nicardipine had an inhibitory effect on M1-receptor binding.