The sodium borohydride reduction of 3,5-disubstituted 1,4-dihydro-2,6-dimethyl-4-(pyridinyl)pyridines 2 and 5 in the presence of methyl, phenyl, or tert-butyl chloroformate afforded the respective 4-(dihydropyridinyl)-1,4-dihydropyridines 4 and 6 in good yield. Products 4 comprised a mixture of the 1,2- and 1,6-dihydropyridinyl regioisomers 4a and 4b where 4a was always the predominant regioisomer. Compounds possessing a 4-[dihydro-1-(phenoxycarbonyl)-3-pyridinyl] substituent, such as 26, were also a mixture of two regioisomers 26a and 26b, and each regioisomer existed as a mixture of two rotamers in Me2SO-d6 at 25 degrees C (26a', 26a'', and 26b', 26b'') due to restricted rotation about the nitrogen-to-carbonyl carbamate bond. The calcium antagonist activities for 4 and 6 were determined by using the muscarinic receptor-mediated Ca2+-dependent contraction of guinea pig ileal longitudinal smooth muscle. The relative order of activities for the 4-(dihydropyridinyl) analogues was 4-(dihydro-3-pyridinyl) greater than 4-(dihydro-4-pyridinyl). Increasing the size of the C-3(5) alkyl ester substituents increased activity. Compounds having nonidentical ester substituents were more active than those having identical ester substituents. Replacement of the C-3 and/or C-5 ester substituents by a cyano substituent(s) decreased activity significantly. An approximate 1:1 correlation between the IC50 value for inhibition of [3H]nitrendipine binding and inhibition of the tonic component of the muscarinic-induced contractile response was observed. The test results suggest that a 4-(dihydropyridinyl) substituent is bioisosteric with a 4-(nitrophenyl) substituent on a 1,4-dihydropyridine ring where m- and p-nitrophenyl are bioisosteric with the 4-[1,2(1,6)-dihydro-3-pyridinyl] 4 and 4-(1,2-dihydro-4-pyridinyl) 6 isomers, respectively.