The rotational energy relaxation (T-1 processes) of polar diatomic molecules diluted in nonpolar liquids is analyzed by means of a non-Markovian theory for the energy time autocorrelation function that does not require the usual population-coherence decoupling approximation. Non-Markovian rate equations are obtained in terms of two-time conditional probabilities and the involved transition rates are calculated in terms of quantum time correlation functions associated to the solute-solvent interaction. Alternative time scales for the discrete rotational levels have been introduced and compared with previous definitions. The usual long time, Markovian limit is recovered. The theory is applied to the study of the rotational energy relaxation of HCl in liquid SF6.