The glycaemic and lipaemic effects of 3 feeding regimes and 2 post prandial intervals were determined at rest and during exercise using 6 Thoroughbred geldings on 6 testing days according to a Latin square design. Three feeding regimes consisted of a fasting regime and isoenergetic meals (4.1 Mcal) of either cracked corn or alfalfa. One or 4 h after feeding, a standardised exercise test (SET) was imposed on fed and fasted horses. Blood was sampled via jugular catheters every 15 min from 0630 to 1500 h, except during the 55 min SET in which 7 samples were drawn. The SET consisted of three 10 min bouts of saddling, walking to the arena and warm-up (walking and trotting), followed by 3 bouts of progressively more intense cantering at heart rates of 130–140 (10 min), 150–160 (10 min) and 170–180 (5 min) beats/min. There were no feeding regime effects on pre-feeding levels of glucose, insulin, lactate, glycerol or free fatty acids (FFA). However, glucose in corn fed horses fell below pre-feeding concentration during the SET but rebounded at the termination of cantering. Glucose in alfalfa fed and fasted horses gradually rose throughout the SET. Insulin levels closely followed glucose response. During the SET, corn fed horses had higher insulin concentrations (P<0.05) than fasted or alfalfa fed horses. During the onset of the SET one or 4 h post prandially, FFA levels in corn fed horses were less (P<0.05) than in alfalfa fed or fasted horses. Glycerol levels gradually increased during the SET; however, no differences were found between dietary treatments.