Mechanical threshold was measured "in vitro" in extensor digitorum longus (EDL) muscle fibers from rats of 3-4 and 29 months of age, by means of a two microelectrode "point" voltage clamp. The potential needed for evoking a barly visible contraction was determined using depolarizing command pulses of 5-500 ms duration. At each pulse duration, the EDL fibers from aged rats contracted at a significantly more negative potential than did those from the younger adult rats. Accordingly, the strength duration curve of the aged EDL was significantly shifted towards more negative potentials compared to that for adult rats. The rheobase voltages estimated from the fit of such curves were -62.6 +/- 0.81 mV and -57.1 +/- 0.87 mV in aged and adult EDL fibers, respectively. The data suggest that changes in excitation-contraction coupling parallel the prolongation of contractile times observed during aging in mammalian skeletal muscle. These results are consistent with the known reduction in rate and extent of Ca++ uptake by sarcoplasmic reticulum in aged rats.