This study was designed to quantify the effect of dietary manganese deficiency on rat hepatic Mn concentration and arginase activity. Weanling male Sprague-Dawley rats were randomly assigned to two groups of nine rats each and fed L-amino acid diets with 0 or 48 micrograms Mn/g diet for 21 d. After 21 d, hepatic Mn concentration (mumol/g liver dry wt, mean +/- SEM) was 0.130 +/- 0.005 for the control group but was lower (P < 0.01) in the Mn-deficient group (0.040 +/- 0.003). There were no differences in the hepatic concentrations of any other measured mineral. Hepatic arginase activity [mmol ornithine/(g hepatic protein.min)] was 1.55 +/- 0.22 in the control group and was lower (P < 0.01) in the Mn-deficient group (1.12 +/- 0.26). Plasma ammonia concentration was 301 +/- 6 mumol/L in the control group and was higher (P < 0.01) in the Mn-deficient group (480 +/- 8 mumol/L). In contrast, plasma urea concentration was higher (P < 0.01) in the control group (350 +/- 10 mumol/L) than in the Mn-deficient group (267 +/- 7 mumol/L). There were no differences in plasma concentrations of arginine or other amino acids between the control and the Mn-deficient groups. Our results demonstrate for the first time that dietary Mn deficiency results in altered plasma concentrations of ammonia and urea in association with decreased hepatic Mn concentration and arginase activity in young growing rats.