We reanalyze the theoretical lifetimes of the metastable a (3)Pi (v=0,Omega,J) levels as well as the spontaneous a (3)Pi(v = 0) decay rates into the X (1)Sigma(+) (v) ground state levels, clearly favoring the measurements of Jongma et al. [J. Chem. Phys. 107, 7034 (1997)] as compared to the theoretical results of James [J. Chem. Phys. 55, 4118 (1971)]. Theoretical lifetimes of the vibrationally higher lying metastable levels a (3)Pi (v=1-3,Omega,J) are listed for the first time. The lifetimes of the two Lambda components are shown to be different. A new technique for measuring the lifetime of individual rovibrational metastable molecular levels is applied to the CO rovibrational level a (3)Pi (v = 3,Omega =1,J=2). Our result tau=3.04+/-0.38 ms is in good agreement with the theoretical lifetime of 2.97 ms. The metastable population is deduced to be N-meta =(2.3+/-1.2) . 10(6) molecules/shot. Finally, our ms lifetime measurement technique is compared with that developed by Jongma et al. [J. Chem. Phys. 107, 7034 (1997)].