The aim of this work was to test the hypothesis that in vivo carnosine biosynthesis is dependent upon endogenous beta-alanine availability, by studying the effect of sustained dietary beta-alanine supplementation in the horse on the carnosine concentration in types I, IIA and IIB skeletal muscle fibres. The diets of 6 Thoroughbred horses were supplemented 3 times/day with beta-alanine (100 mg/kg bwt) and L-histidine (12.5 mg/kg bwt) for a period of 30 days. Percutaneous biopsies of the m. gluteus medius from a depth of 6 cm were taken on the days immediately before and after the supplementation period. Heparinised blood samples were collected at hourly intervals on the first and last days of supplementation, and on every sixth day during the supplementation period, 2 h after each ration. Individual muscle fibres were dissected from freeze-dried biopsies, weighed and characterised histochemically. beta-alanine, histidine and carnosine concentrations were measured in plasma. The areas under the plasma concentration-time curves (AUC) for beta-alanine and histidine were calculated as indicators of the doses absorbed. Carnosine concentrations were measured in types I, IIA and IIB muscle fibres. There was an adaptive response to sustained beta-alanine administration resulting in mean +/- s.d. beta-alanine AUC increasing significantly from 1130 +/- 612 mumol/l h (Day 1) to 2490 +/- 1416 mumol/l h (Day 30) (P < 0.05). This was probably due to increased beta-amino acid transport across the gastrointestinal lumen. There was no consistent increase in histidine AUC between Days 1 and 30, (mean +/- s.d. values being 757 +/- 447 mumol/l h Day 1[ and 1162 +/- 1084 mumol/l h Day 30[ P > 0.05[). Type IIA fibre carnosine concentrations increased from 59.9-102.6 to 76.2-112.2 mmol/kg dry weight (dw). Increases were statistically significant in 2 of the 6 horses (P < 0.05 in both instances). Type IIB fibre carnosine concentrations increased from 101.3-131.2 to 114.3-153.3 mmol/kg dw. Increases were statistically significant in 5 of the 6 horses (P < 0.05 in 3 horses, P < 0.01 in 1 horse, P < 0.005 in 1 horse). Changes in muscle carnosine concentration appeared to be influenced by beta-alanine bioavailability. Individual increases in muscle carnosine concentration were significantly correlated with individual changes in beta-alanine AUC (r2 = 0.973, P < 0.005). Increased muscle carnosine concentrations lead to increased intramuscular hydrogen ion (H+) buffering capacity.