Intracavity laser absorption spectroscopy ("ICLAS") has been demonstrated as a feasible detection method for trace species in a discharge flow tube. This implementation has been used to measure the rate of the reaction between atomic hydrogen and NO to form HNO in helium carrier gas. A reaction rate constant of (4.3 ± 0.4) x 10(-32) cm(6) molecule(-2) s(-1) at 295 K was measured for the reaction H + NO + M → HNO + M (M = He). The pressure and concentration range enabled by ICLAS detection has allowed us to limit reactive pathways that would inhibit the formation of HNO. The sensitivity of ICLAS, coupled with the versatility of the discharge flow technique, suggests that intracavity absorption spectroscopy will be a useful technique for kinetics measurements on free radicals and other reactive species.