Structural and conformational properties of 1,4-dihydroisonicotinic acid derivatives, characterized by ester, ketone or cyano functions at positions 3 and 5 in solid and liquid states have been investigated by X-ray analysis and nuclear magnetic resonance and supported by quantum chemical calculations. The dihydropyridine ring in each of the compounds exists in flattened boat-type conformation. The observed ring distortions around the C(4) and N(1) atoms are interrelated. The substituent at N(1) has great influence on nitrogen atom pyramidality. The 1H, 13C and 15N NMR chemical shifts and coupling constants are discussed in terms of their relationship to structural features such as character and position of the substituent in heterocycle, N-alkyl substitution and nitrogen lone pair delocalization within the conjugated system.