The aim of this study was to relate data obtained in lactating dairy goats to those obtained using a short term in vitro method. Two experiments were conducted: in the first, four groups of 8 goats received one diet containing either lucerne hay or maize silage as a forage and a concentrate either rich in starch or rich in cellwalls. In the second, ammonia and volatile fatty acid production was measured after 6 hours of in vitro incubation of each diet ingredient. In vitro results were used to calculate by additivity theoretical ammonia concentrations and intake and available carbon fluxes for each goat. Uraemia was correlated with the calculated ammonia concentration, and urinary nitrogen excretion was more closely related to calculated ammonia intake than to uraemia. The carbon flux available for de novo milk fatty acid synthesis estimated from in vitro C2 and C4 VFA production was highly correlated to in vivo C flux incorporated into short and medium chain milk fatty acids. Thus, de novo milk fatty acid synthesis and nitrogen excretion might be predicted from in vitro data.