Laser pulses were applied to granular, and c-axis oriented, YBa2Cu3Ox films current-biased in a resistive state, and the decay of the transient voltage was monitored as a function of time. At low enough temperatures and fluences (approximately equals 1 nJ per cm2 per pulse), the decay rate follows a T3-dependence characteristic of electron energy loss to acoustic phonons. Above about 7 K, the response time of 300 angstroms films stays constant at 2.4 nsec, in agreement with the bolometric response observed by others. In the range of dominant electron-phonon interaction, the response time contains direct information about the coupling constant (lambda) , via a formula derived by P.B. Allen. However, as in ultrasonic attenuation, the limitation of the electron mean free path must be taken into account. A support for this procedure is the approximate proportionality of the relaxation time upon the room temperature resistivity, i.e. the electron mean free path. We thus obtain a value of (lambda) appropriate to the acoustic mode interaction.