Anecdotal evidence suggests that a horse’s prior fitness level has an effect on the horse’s ability to return to that previous level of fitness. This trial aimed to test this assumption which would benefit horse trainers to create individual fitness programs for horses. Standardised exercise testing was used to compare the progress of two groups of horses during weeks three and seven of a 14-week training program. Group A had undergone a similar training program twelve months previously and Group B had not. Plasma lactate samples and heart rate monitoring were used to assess the levels of fitness of the horses. Analysis of variance showed that there was no significant difference in post-exercise plasma lactate concentration between the groups. The horses with previous training experience (Group A) had lower (115.7 v 130.6 bpm) but non-significantly different heart rates than those without previous training experience (Group B) at week 3. This trend did not change after an additional 4 weeks of training (115.5 v 128.4 bpm; Group A and B respectively). Irrespective of training history or speed of each incremental step, there was no improvement in heart rate between week 3 (123.1 bpm) and week 7 (122.0 bpm).