Experiments were conducted with chronically catheterized growing lambs to study the effect of variations in rumen volatile fatty acid (VFA) supply on blood plasma free amino acids (FAA) and insulin. Five male lambs were fed, then fasted for 72 h and refed. In the second experiment 48 mmol of a VFA mixture per kg body weight was infused into the rumen vein of another 5 lambs. Rumen VFA, plasma FAA and immunoreactive insulin (IRI) concentrations were determined over the 72 h postfeeding period and during 24 h after refeeding as well as over 24 h after the VFA infusion. Both postprandial increase of rumen VFA and VFA infusion caused a significant increase of plasma FAA concentrations and an increase in IRI. Feed intake decreased the prefeeding total FAA (TFAA) concentration (2.7 mmol/l) to 1.8 mmol/l at 4 h after feeding. It rose again to 2.5 mmol/l at 24 to 72 h postfeeding. IRI seemed to have the highest concentrations when the TFAA were the lowest. VFA infusion also decreased the plasma TFAA, to about half of the preinfusional value. IRI, however, showed a five-fold increase after infusion. The results of these experiments show that variations in VFA absorption from the rumen due to different nutritional stages result in changes of the plasma FAA concentrations. Insulin seems to have an important role in the control of these changes.