Non-spinning syndrome in Bombyx mori (L., 1758) (Lepidoptera: Bombycidae) is a serious issue for the sericulture industry. Determination of urea metabolism as an important parameter at the onset of spinning has shown the need for examining the role of urea metabolism in the non-spinning syndrome. The aim of this study was to investigate role of urea metabolism in the non-spinning syndrome by evaluating urease activity and L-arginine concentrations in the silkworm hemolymph and mulberry leaves. Additionally, urea concentrations were determined in hemolymph samples. Urease activities in hemolymph samples were almost twice as high in spinning larvae (SL) than in non-spinning larvae, 25 +/- 5.8 vs 10.9 +/- 2.4 units/l (P < 0.05). Urea concentrations in the SL hemolymph decreased significantly from day 5 (137 +/- 13 mg/l) to day 7 (97 +/- 17 mg/l) of the fifth instar (P < 0.01), it remained almost constant in NSL hemolymph (149 +/- 19 to 167 +/- 4 mg/l). Additionally, L-arginine concentrations in hemolymph samples obtained from NSL of 4.55 +/- 0.48 mM were significantly higher than in SL at 2.72 +/- 0.45 mM (P < 0.01). Changes in urease activity and L-arginine concentrations in hemolymph were similarly observed in mulberry leaves. These results suggested that changes in urea metabolism may cause or contribute to non-spinning syndrome in silkworms.