The theory of electronic-vibro-rotational radiative transitions is considered within the framework of the perturbation theory. For the case of regular (monotonic) perturbations caused by relatively far lying terms of the molecule the simple analytical expressions for the line strengths of the^{2 s+1}A′→^{2 s+1}A″ transitions are derived in the second order of the perturbation theory. Most distributed schemes of angular momenta coupling - Hund's cases "a" and "b" - are considered. It is shown that the line strengths can be expressed in terms of a finite number (usually small) of the parameters (certain combinations of matrix elements describing the effects of perturbations and vibration-rotation interaction) which may be determined either from the experimental data in a semiempirical approach or from numerical calculations of adiabatic electronic and vibrational wave functions.