A previously published report indicated that there are early post mortem changes in the pulsed NMR proton spin-lattice relaxation time (T1) of skeletal muscle which must be taken into account in in vitro tissue analysis. We re-examined this subject. When T1 measurements were done by allowing the signal intensity to decay over two orders of magnitude from the original intensity, the T1 decay curves showed more than a single exponential relaxation time component as reported earlier. However, when T1 measurements were done by allowing signal intensity to decay to about one order of magnitude from the original intensity only a single exponential relaxation time component was found. We postulate that the latter method gives the average T1 value of the various macroscopic tissue components present and that this method gives a representative T1 value for the entire tissue. Using data obtained in this manner we could not find significant post mortem changes in the muscle T1 relaxation times during the first four hours but found change at a later time.