Fluorescence decay times of the nucleosides: adenosine, guanosine, cytidine and thymidine, and of the corresponding nucleotides, were determined using the technique of fluorescence up-conversion with femtosecond time resolution. The excited-state lifetimes of these nucleic acid molecules all fall in the sub-picosecond time scale, confirming the presence of an ultrafast internal conversion channel for both the nucleotides and the nucleosides; the nucleotides lifetimes are longer than those of the nucleosides by up to 20%. The ultrafast internal conversion is biologically relevant to the stability of DNA, and our results support the sub-picosecond repopulation of the ground state, consistent with transient absorption studies on the femtosecond time scale.