Ultrafast excited state lifetimes of the DNA and RNA bases have been observed experimentally. Theoretically these lifetimes can be explained by radiationless decay and fluorescence quenching facilitated by conical intersections (actual potential energy surface crossings). In this work we present ab initio studies of the pyrimidine bases, uracil and cytosine, and several of their analogs in an effort to correlate the molecular structure to the photophysical behavior. Our results show the presence of many seams of two- or three-state conical intersections in both types of systems and energetic differences seem to be the cause of the different photophysical behavior.