New synthetic substituted hexahydropyridoindoles were studied for their radical scavenging ability in a system of an ethanolic solution of alpha,alpha'-diphenyl-beta-picrylhydrazyl and for their lipid peroxidation inhibitory properties in a suspension of unilamellar dioleoylphosphatidylcholine liposomes. The activities in both in vitro systems were correlated with several structural parameters. In the homogeneous system of alpha,alpha'-diphenyl-beta-picrylhydrazyl, the sum of aromatic substitution constants (sigma(+)) and the hydration energy were shown to be effective predictors of the radical scavenging activity of the hexahydropyridoindole derivatives. Moreover, in the heterogeneous system comprising a model liposomal membrane, the overall antioxidant activity of the compounds was affected by their lipid-phase availability governed by the lipophilicity and basicity of the molecules.