The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of 2,2'-dipyridyl (DP) and N-acetyl histidine (HisH) in aqueous solution have been studied using laser flash photolysis and time-resolved CIDNP techniques. The triplet state (DP)-D-T reacts with protonated HisH(2)(+) via hydrogen atom transfer with a rate constant k(H) = 1.2 x 10(8) M-1 s(-1), and with deprotonated His(-) via electron transfer with k(e) = 7.5 x 10(9) M-1 s(-1). No reaction occurs when the histidine imidazole ring is in its neutral state HisH, or when the dipyridyl triplet is protonated, (DPH+)-D-T. The nuclear spin-lattice relaxation times in the radicals formed in these reactions have been determined from the CIDNP kinetics: T-1 = 44 +/- 9 mu s for all DP protons, T-1 = 196 +/- 25 mu s for the beta-CH2 protons of HisH, and T-1 = 16 +/- 5 mu s for the H-2 and H-4 protons of HisH. Under strongly basic conditions the CIDNP is greatly affected by degenerate electron exchange between the neutral His radical and His- anion, with rate constant k(ex) = 1.5 x 10(8) M-1 s(-1).