The general expressions we previously derived for calculating internal conversion rate constants between two adiabatic displaced-distorted-rotated potential energy surfaces, by inducing all vibratinal modes, are applied to estimate the decay rate constants of 1(1)B(u) (sic) 2(1)A(g) and 2(1)A(g) (sic) 1(1)A(g) internal conversions in trans,trans-1,3,5,7-octatetraene molecule. The minimal models with respect to the number and types of vibrational modes are determined for these processes. Our calculations show that in the low temperature limit the 1(1)B(u) (sic) 2(1)A(g) internal conversion takes place on a 232-290 fs time scale in the condensed phase and 2 ps in the gas phase, whereas 2(1)A(g) (sic) 1(1)A(g) internal conversion takes place on a 2 mu s time scale under the isolated conditions.