Results of studies of static nonlinear dielectric effect (NDE) in the isotropic phase in the homologous series of nDBT: (5-n-alkyl-2-(4-isothio-cyanatophenyl)-1.3-dioxane) for n= 4, 6 and 8 exhibiting the I-SmA transition are presented, The strong pretransitional anomaly in the isotropic phase is similar to the one observed in nematogens. It has been found that the discontinuity of the I-N transition increases from 10.8 K to 18.8 K when moving from 4DBT to 8DBT, It has been also shown that pressure reduces the discontinuity of the I-SmA transitions. Results obtained are compared with similar studies in two other homologous series.