The results of dielectric studies of 5-n-hexyl-2-(4'-isothiocyanato)-l,3-dioxane (6DBT) in the smectic A, phase at pressures up to 150 MPa and temperatures up to 340 K are presented. The low frequen-cy relaxation time τ|| (p, T) yields the activation volume Δ# V|| = RT(∂|| In τ||/p)T and activation enthalpy Δ# H|| = R(∂In τ||/∂T-1)p. The calculated values of these parameters are compared with those ob-tained recently for n-octyl-cyanobiphenyl (8CB) forming the smectic Ad phase. In the case of 6DBT both these quantities are practically constant, whereas those for 8CB decrease, when going away from the phase transitions isotropic -smectic A1 (6DBT) or nematic -smectic Ad (8CB). These differences support our earlier conclusion that increase of pressure leads to a breaking of the antiparallel associa-tions of cyanobiphenyl molecules in the smectic as well as in the nematic phases.