Feeding a low-protein diet can enhance urea reabsorption and transfer to the rumen, which might alter microbial protein (MCP) synthesis and redirect molecular hydrogen (H-2) flow away from methanogenesis. The aim of this study was to determine the effects of feeding goats a low-protein diet on urea nitrogen (N) salvage and metabolism, and consequence on H-2 metabolism for MCP synthesis and methanogenesis. Goats (Xiangdong Black goats, a local breed in southern China,) with body weight of 20.4 +/- 2.15 kg (mean +/- SD) were fed a control diet (consisted of 70% rice straw and 30% concentrates, with crude protein [CP] content by 108 g kg(-1)) or a low-protein diet (55.2 g CP/kg). (NH4Cl)-N-15 and H-2(2) were employed to further confirm the incorporation of N and hydrogen for MCP synthesis through in vitro culture. Feeding a low-protein diet resulted in greater proportion of retained N (+156%, P < 0.05), plasma urea N (+81.9%, P < 0.05) or rumen ammonia-N (+125%, P < 0.001) relative to digested N and greater renal urea N reabsorption (+11.2%, P < 0.05), together with upregulated relative protein expression of urea transporter A (UTA) in the kidneys (P < 0.05) and mRNA expression of UTA in rumen epithelium (P < 0.05). Feeding low-protein diet resulted in greater concentration of rumen dissolved hydrogen (+55.6%, P < 0.05), ratio of MCP to ammonia-N (+17.6%, P < 0.05), and lower enteric methane emissions (g/kg digested DM, -18.5%, P < 0.05). Further, in vitro experiments verified that the addition of NH4Cl decreased methane and hydrogen production (P < 0.001), although only a minor quantity of headspace H-2(2) gas was incorporated for MCP synthesis. In conclusions, feeding a low-protein diet to goats enhances renal urea reabsorption capacity and facilitates the rumen microbial hydrogen incorporation for MCP synthesis, leading to enhanced N retention efficiency and reduced methane emissions. However, the effect of such an approach on livestock performance should be tested in the future studies.