Excited states in double-stranded oligonucleotides containing G.C base pairs were studied by femtosecond transient absorption spectroscopy. Relaxation to the electronic ground state occurs about 10 times more slowly in the duplexes and hairpins studied on average than in the individual mononucleotides of G and C. Detection of long-lived excited states in G.C oligonucleotides complements the earlier observation of slow ground-state recovery in A.T DNA, showing that excited states with picosecond lifetimes are formed in DNAs containing either kind of base pair. The results show further that Watson-Crick G.C base pairs in these base-paired and base-stacked duplexes do not enable subpicosecond relaxation to the electronic ground state. A model is proposed in which fluorescent exciton states decay rapidly and irreversibly to dark exciplex states. This model explains the seemingly contradictory observations of femtosecond fluorescence and slower, picosecond recovery of the ground-state population.