Two experiments were conducted to determine the effects of vitamin C supplementation 48 h before slaughter on plasma ascorbic acid and oxalate concentrations and its effect on pork quality. In Exp. 1, 16 pigs (87.8+/-2.13 kg BW) were blocked by sex and weight and assigned randomly within block to one of three vitamin C treatments: 1) control; 2) 1,000 mg/L; or 3) 2,000 mg/L supplemented in the drinking water for a 48-h period. This was then followed by an additional 48-h period without supplemental vitamin C. Vitamin C increased plasma ascorbic acid concentrations (11.6, 19.5, and 23.4 microg/mL for 0, 1,000, and 2,000 mg/L of vitamin C; P < 0.05) within 6 h of supplementation. Plasma ascorbic acid concentrations from treated pigs decreased and did not differ from those of control pigs (13.7, 18.2, and 18.6 microg/mL for 0, 1,000, and 2,000 mg/L of vitamin C; P = 0.30) within 2 h of ending supplementation. No differences in plasma ascorbic acid concentrations were found between the two levels of supplementation. Vitamin C did not affect plasma oxalate or cortisol; however, cortisol tended to increase quadratically (P = 0.077) with vitamin C after 96 h. In Exp. 2, 30 pigs (107.5+/-0.54 kg BW) were blocked by sex and weight and assigned randomly within block to one of three vitamin C treatments: 1) control; 2) 500 mg/L; or 3) 1,000 mg/L supplemented in the drinking water 48 h before slaughter. Pigs were slaughtered 4 to 5 h after vitamin C supplementation ended, and loin samples were collected for meat quality measurements. At the time of slaughter, no differences in plasma ascorbic acid or cortisol were observed, but oxalate tended (P = 0.074) to increase quadratically with increasing vitamin C. Muscle ascorbic acid at slaughter and lactic acid in muscle at 0 and 1.5 h after slaughter were not different; however, lactic acid increased (P = 0.048) quadratically at 24 h after slaughter. Vitamin C did not affect initial or ultimate pH. Initial fluid loss (P = 0.041), and fluid loss on d 4 (P = 0.014) and 8 (P = 0.076) of simulated retail display; L* on d 0 (P = 0.038), 4 (P = 0.010), and 8 (P = 0.051); a* on d 0 (P = 0.021); and b* on d 0 (P = 0.006), 4 (P = 0.035), and 8 (P = 0.017) were negatively affected in a quadratic manner when vitamin C was supplemented. Vitamin C tended (P = 0.086) to increase oxidation in chops on d 0, but not d 4 or 8. Results indicate that on-farm supplementation of vitamin C was generally not effective in improving pork quality, which may be related to timing relative to slaughter.