Two novel radioactive 1.4-dihydropyridine derivatives were synthesized and their properties were shown to be characteristic for calcium antagonists of nifedipine series. Photoactivable o- and p-azidobenzoyl groups as well as one or two tritium atoms were introduced into the reactive site of these compounds. The binding of molecular probes obtained with rabbit skeletal muscle membranes was characterized by a high level of specificity, the affinity constant being about 10(-9) M. Upon UV-light photoactivation, the radioactive probes selectively incorporated into skeletal muscle membrane protein (Mr approximately 136 kD).