The bioavailability of ascorbic acid administered to thoroughbreds by intramuscular injection was investigated. For intramuscular injection two preparations were studied, and the percentage bioavailability up to 24 h of 10 g of ascorbic acid was 95% +/- 22 in four horses and 60% in two horses with preparations A and B, respectively. Bioavailability at 24 h in three horses injected subcutaneously with 10 g of preparation B was 82%. Intramuscular injection of both preparations was apparently well tolerated while subcutaneous injection of preparation B (pH 6.0) was associated with marked irritancy. In a cross-over trial in seven thoroughbreds the effect of 13 or 15 days of oral administration of crystalline ascorbic acid (20 g) or ascorbyl palmitate (47 g) on plasma ascorbic-acid concentrations was investigated. Marked differences occurred between individuals. There was a greater increase in plasma ascorbic-acid concentration with ascorbyl palmitate compared to ascorbic acid at 6 and 24 h following administration. In two horses there was no increase in plasma ascorbic acid at 6 h following either oral preparation. The finding of lowered plasma ascorbic-acid concentrations following a period of supplementation warrants further investigation to assess its significance.