The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pipi* state to an npi* state (lifetime tau(npi)* approximate to 100 fs) followed by a slower decay of the latter with tau(npi)* 0.9 ps for (adenine)(2), tau(npi)* = 6-9 ps for (thymine)(2), and tau(npi)* approximate to 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pipi* state via the pidelta* state greatly suppressed the npi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.