The pulsed-laser-photolysis technique was used to determine the rate constant of the gas-phase reaction between the hydroxyl radical (OH) and HNO3 for the first time under conditions of high relative humidity at (295 +/- 3) K. The value obtained, at a total pressure of 200 Torr N-2, was 1.64(-0.20)(+0.11) x 10(-13) cm(3) s(-1) (error limits include assessment of systematic errors), in excellent agreement with the most recent determinations [J. Phys. Chem. 103 (1999) 3031], and was invariant with relative humidity up to 0.50. The shape of the HNO3 absorption bands between 210 and 350 nm also showed negligible Variation with relative humidity.