Training programmes are used routinely to improve fitness and thus prevent sport related injuries in horses. Little is known about the effect of training programmes in Icelandic horses. In this study, response to training was evaluated using standardised exercise tests (SET) before and after an 8-week interval training programme with increasing intensity. 22 Icelandic horses were included of which 14 horses completed all parts of the study. SETs were performed ridden on the same oval track and included four steps in different gaits and speeds including two steps in tölt. For SET2 a third step in tölt was added at the end. Heart rate (HR) and speed (v) were measured continuously during both SETs, and V140 and V180 were calculated for tölt. Blood lactate concentration was measured after step four in both SETs. Horses were divided into two fitness groups (‘low’ and ‘high’) based on blood lactate concentrations in SET1. The ‘low’ group had blood lactate concentrations >4 mmol/l. V140 (speed at HR140) and V180 were equal between groups in SET1. Interval programmes were adjusted to each fitness group to avoid injury to less fit horses. After 8 weeks of training, blood lactate concentration decreased significantly for the ‘low’ group (P = 0.002), and there was no longer a difference between groups. Between SET1 and SET2, V180 increased significantly for the ‘high’ group (P = 0.04). In conclusion, the interval training programme resulted in a decrease in blood lactate concentration in the ‘low’ group and an increase in V180 in the ‘high’ group suggesting an improved fitness level and cardiovascular capacity.