Two different techniques were applied to measure the lifetime of the lowest rotational levels in the metastable a (3) Pi(1)(v=0) state of CO. First, measurement of the absolute absorption cross-section for several absorption Lines of the a(v=0)<--X(v=0) transition yields an Einstein coefficient of A(0,0)=97+/-3 s(-1). In combination with the experimentally determined branching ratios for the a-->X transition, the lifetime of each component of the a (3) Pi(1)(v=0,J=1) n-doublet is determined to be 3.67+/-0.20 ms. Second, detection of the spin-forbidden fluorescence at two positions in the molecular beam downstream from the excitation region, as a function of velocity of the molecules directly probes the exponential decay. With this technique the Lifetime of the lower component of the same a (3) Pi(1)(v=0,J=1) Lambda-doublet is determined to be 3.4+/-0.4 ms, while for the upper component a value of 3.8+/-0.5 ms is found. (C) 1997 American Institute of Physics.