We evaluated the use of urinary sulfur (S) excretion as a measure of sulfur amino acid (SAA) catabolism and the nitrogen/sulfur (N/S) molar balance ratio as an indicator of nonprotein SAA storage in growing piglets. After confirming that an intravenous dose of sulfate is fully recovered in urinary sulfate, we measured urinary S recovery after an intravenous dose of methionine in 6 piglets fed an adequate protein (AP) diet and 6 piglets fed a low protein (LP) diet with normal energy provision. As measured over 48 h, recoveries of the methionine load as urinary total S was 106% in the AP group but only 69% in the LP group (P < 0.05). On the baseline diets the N/S balance ratio in the AP group was 36, whereas that in the LP group was 30 (P < 0.05); immediately after the methionine load, this ratio remained constant in the AP group but decreased further, to 26 (P < 0.05) in the LP group. These results indicate that protein-deficient piglets accumulate relatively more S than N from their diet, and under these conditions a significant portion of the S derived from a methionine load is retained in nonprotein compounds. Urinary S excretion, a simple nontracer measurement, can provide an accurate measure of SAA catabolism, and the N/S balance ratio is a potentially useful indicator of changes in nonprotein SAA stores of growing piglets.