The post-natal changes in resting muscle tension were investigated at 20°C using small muscle fiber bundles isolated from either the edl or the soleus of both neonatal (7-21 days old) and adult rats. The results show that the tension-extension characteristics of the bundles depended on the age of the rats. For example, both the edl and soleus bundles of rats older than 14 days showed characteristic differences that were absent in bundles from younger rats. Furthermore, the tension-extension relation of the adult slow muscle fibers were similar to those of the two neonatal muscles and were shifted to longer sarcomere lengths relative to those of the adult fast fiber bundles. Thus, at the extended sarcomere length of 2.9µm the adult fast muscle fiber bundles developed higher resting tensions (5.6±0.5 kN m^-2) than either the two neonatal fiber types (~3 kNm^-2) or the adult slow muscle fibers (3.1±0.4 kNm^-2). At all ages examined, the resting tension responses to a ramp stretch were qualitatively similar and consisted of three components, a viscous, a visco-elastic and an elastic tension. However, in rats older than 14 days all three tension components showed clear fast/slow fiber type differences that were absent in younger rats. Bundles from 7-day old rats also developed significantly lower resting tensions than the corresponding adult ones. Additionally, the resting tension characteristics of the adult fibers were not affected by chemical skinning. From these results we conclude that in rats resting muscle tension, like active tension, differentiates within the first 3 weeks after birth.