The gas-phase association reactions of ground-state manganese, iron, and ruthenium atoms with nitric oxide are reported. The transition metal atoms were produced by the 248 nm photodissociation of 2-methylcyclopentadienylmanganese tricarbonyl, ferrocene, and ruthenocene. Detection of these transition metal atoms was by laser-induced fluorescence. Manganese was found to be the least reactive with nitric oxide of the transition metals in this work. The limiting low-pressure third-order rate constant, k(o), of Mn + NO + Ar can be expressed as (9.5 +/- 2.5) x 10(-33) exp[-1.5 +/- 0.4 kcal mol(-1)/(RT)] cm(6) molecule(-2) s(-1). k(o) for Fe + NO + N-2 at 296 K is (2.3 +/- 0.5) x 10(-32) cm(6) molecule(-2) s(-1); this reaction is independent of temperature over the range 296-622 K. Ruthenium was found to be the most reactive toward NO. In Ar buffer, k(o) (7.3 +/- 1.0) x 10-30 cm(6) molecule(-2) s(-1) and the limiting high-pressure rate constant k(infinity) = (3.8 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1) for a fixed broadening factor of F-c = 0.6. The uncertainties here represent precision. Recent density functional theory calculations on transition metal mononitrosyls were combined with RRKM calculations to estimate the binding energies of the Mn-NO and Fe-NO adducts.