Abstract Three rumen cannulated lactating goats fitted with a temporary polyvinyl intravenous catheter were used to study [ 14C] incorporation in milk proteins (casein (CN), whey proteins (WP)) in comparison with incorporation in lactose (LAC) and in milk fat (MF) after a single ruminal supply of d-[U- 14C]glucose ([ 14C]GLU) (9.2 MBq/goat). Rumen fluid and venous blood samples were collected at 0.5 h, 1 h, 1.5 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 24 h, 36 h, 48 h, 60 h, 72 h, and 84 h; and milk samples were taken at 6 h, 12 h, 24 h, 36 h, 48 h, 60 h, 72 h, and 84 h after the tracer administration. As expected, radioactivity followed a logarithmic decline in total rumen fluid. In plasma and in milk, highest [ 14C] labelling was observed at 0.5 h and 6 h, respectively. [ 14C]-enrichment curves in CN, in WP, in LAC and in MF were similar and the measured radioactivity levels were in the same range. [ 14C]-labelling peak was observed either at the first milking for LAC (3750 mBq/mg C), for CN (2048 mBq/mg C) and for WP (1334 mBq/mg C), or at the second milking for MF (2101 mBq/mg C). Approximately 1% of the ruminal-administered radioactivity was found in milk proteins. Recovery in milk proteins was similar to recovery in MF and represented about 30% of radioactivity obtained in LAC. Thus, end products of glucose metabolism in the rumen are used in relatively high amounts for protein synthesis. Further experiments should now be carried out to specify the associated biochemical pathways.