A twelve-week nutrition trial was carried out to evaluate the efficacy of different carbohydrate levels on digestibility, feed intake, growth performance, carcass and muscle composition and plasma glucose level in the rainbow trout, Oncorhynchus mykiss. Three test diets were formulated using extruded wheat meal (15.3, 32.2 and 43.5%) as the carbohydrate source and fed rainbow trout (Initial Body Weight: 33.8 +/-0.46 g.). These were either restricted: LCR (Low Carbohydrate Restricted), MCR (Medium-Carbohydrate-Restricted) and HCR (High-Carbohydrate Restricted); or satiation: LCS (Low-Carbohydrate Satiation), MCS (Medium-Carbohydrate Satiation) and HCS (High-Carbohydrate Satiation) respectively. The aim of applying different feeding regimes was to quantify the protein sparing effect of carbohydrate and to determine the response of rainbow trout to different levels of carbohydrates. The results showed that protein, energy and carbohydrate digestibility was reduced with increasing dietary carbohydrate levels, whilst lipid digestibility was similar all the groups. There was a significant energy substitution from the carbohydrate source in the restricted groups. However, since the maximum feeding rate was reached in the restricted regimes (MCR and HCR), a similar sparing action of carbohydrate was observed in the satiation groups (MCS and HCS). All the groups displayed good growth performance. and exhibited growth that was LCS exhibited the highest (P<0.05) growth rate. MCR and MCS trout exhibited growth that was superior (P<0.05) to that of the LCR, HCR and HCS treatments. The feed efficiency of all the treatments was close to 100%. The quantities of carcass and muscle protein, lipid and ash were not found to be different (P>0.05). Transient hyperglycaemia was observed in trout fed a high carbohydrate diet. These results are discussed with respect to the protein sparing action of dietary carbohydrate in practical trout diets and the possible physiological effects of carbohydrates on feed intake regulation.