The purpose of the present study is to determine whether or not the differences in cellular proliferation in rat nasal septal cartilage and tibial cartilage are reflected by differences in the pattern of age-related changes in thymidylate synthetase (TS) and thymidine kinase (TK) activities. TS and TK are responsible for thymidine monophosphate formation via, respectively, the de novo and the salvage pathways. The present study also examines the effects of glucocorticoids, which are known to affect the proliferation of chondrocytes, on TS and TK activities in these cartilaginous tissues. Both enzyme activities declined with age, and the decline in TK activity was more rapid than that of TS. The TK/TS ratio decreased with age more rapidly in nasal septal cartilage. These findings suggest that nasal septal cartilage might mature earlier than tibial cartilage and, furthermore, that in rapidly growing tissues the salvage pathway functions predominantly. Prednisone injection (2.5 mg/100 g, at 4 days after birth) clearly disturbed the gain of body weight. TK activity significantly decreased concomitantly with this arrest in weight gain, while in contrast TS activity was not depressed. This marked difference between TS and TK activities induced by prednisone suggests that these two enzymes may be regulated independently.