Reasons for performing study: The injury-prone, energy-storing equine superficial digital flexor tendon (SDFT) of the mature performance horse has a limited ability to respond to exercise in contrast with the noninjury-prone, anatomically opposing common digital extensor tendon (CDET). Previous studies have indicated low levels of cellular activity in the mature SDFT, but in foal tendons the tenocytes may still have the ability to adapt positively to increased exercise. Objectives: To measure tenocyte densities and types in histological sections from the SDFT and CDET of horses from controlled long-term, short-term and foal exercise studies. Methods: Specimens were collected from mid-metacarpal segments of the CDET and SDFT for each horse and processed for histology; central and peripheral regions of the SDFT cross-section were analysed separately (SDFTc, SDFTp). Tenocyte nuclei were counted in a total area of 1.59 mm2 for each tendon region in each horse. Each nucleus was classified as type 1 (elongate and thin), type 2 (ovoid and plump) or type 3 (chondrocyte-like); type 1 cells are proposed to be less synthetically active than type 2 cells. Results: No significant differences were noted between exercise and control groups in any of the studies, with the exception of an exercise-related reduction in the proportion of type 1 tenocytes for all tendons combined in the long-term study. There were tendon- and site-specific differences in tenocyte densities and proportions of type 1 and 2 cells in all 3 studies. Conclusions and potential relevance: There was no indication that exercise increased tenocyte density or proportions of the (theoretically) more active type 2 cells in immature horses (short-term and foal studies), perhaps because the training regimens did not achieve certain threshold strain levels. In the foal study these findings can still be interpreted positively as evidence that the training regimen did not induce subclinical damage.