Effects of multivitamin supplementation and type of grain on exercise capability and behaviour was determined in 2 groups of 4 Australian stockhorses. Initially the horses were fed diets containing chopped oaten and lucerne hay and either rolled oats grain or cracked maize. The grains were fed on an isoenergetic basis and contributed 60% of the digestible energy content of the diets. After an 8 week training programme the horses were subjected to 2 standard exercise tests (SET) 2 weeks apart. Each SET was followed, 3 days later, by a run to exhaustion (RTE) at a speed equivalent to 110% of the VO2max recorded during the previous SET. Both diets were then fortified with a complete vitamin mix (except for ascorbic acid and choline) that: supplied vitamins at a level that equalled or exceeded NRC (1989) recommendations. After 2 weeks of supplementation the SETs and RTEs were repeated. Neither grain type nor vitamin supplementation had any significant effect on liveweight, VO2max, VLA4 or total run time to exhaustion. Vitamin supplementation reduced maximum heart rate (mean ± s.e. 227.8 vs. 233.2 ± 1.86 beats/min) and maximum plasma lactate concentrations (11.1 vs. 14.2 ± 0.67 mmol/l) during the SETS. Grain type had no effect on any of the horses' behavioural characteristics. Under the conditions of our experiment neither grain type nor vitamin supplementation affected the performance capability of the horses.