Fluorescence quantum yields and decays have been measured along the rotational contour of the 0-0 band in the S1 ← S0 absorption of pyrazine and pyrimidine in the vapor phase at different pressures. The results are discussed on the basis of a kinetic model and of a quantum-mechanical treatment of the intermediate case with emphasis on the role of rotational levels in S1-T coupling and in collisional quenching of fluorescence. In each molecule, the fluorescence quantum yield spectrum at low pressure exhibits a sharp peak near the band origin. As the pressure increases, the peak becomes less pronounced, and the yield spectrum is rather flat at moderately high pressure, indicating that the marked rotational-level dependence of the quantum yield is attributable mainly to the slow component of fluorescence. The lifetime of the slow fluorescence at low pressure is constant throughout the rotational contour in pyrazine, while it shows a shallow minimum near the band origin in pyrimidine. With pyrazine, the decay data indicate that the ratio between the fluorescence quantum yields of the fast and slow components strongly depends upon the rotational level excited. The number of triplet levels that are coupled effectively to the initially prepared singlet level varies along the rotational contour, with a minimum at the peak of the quantum yield spectrum, and on the whole the number increases with increasing rotational quantum number, J′. It is shown that the rotational relaxation leads predominantly to quenching of the slow fluorescence.