We investigate the effect of an applied field the internal molecular field produced by donor-acceptor groups, as well that of an effective field by the environment, on the electronic properties and geometric structure of substituted polyenes of finite length. We use analytically tractable methods to study (a) thr effect of static field on the bond length alternation (BLA) of finite polyene rings and (b) the effect of static field on even and odd open chains. We find that the field drives the rings to lower BLA and that odd chains in a static field can show reversal of BLA through the equal bond length limit.