The rate of gluconeogenesis in isolated rabbit liver cells has been measured from propionate, lactate, pyruvate and from the combination of propionate at different concentrations either with lactate or pyruvate. The glucose formed from propionate according to its concentration is about 74 to 87% of the glucose formed from lactate. No lag period was observed with preincubated cells and gluconeogenesis is linear at least from 90 min when the substrates are present at a concentration of 5 micrometers or more. Combinations of lactate + propionate increase the glucose formed as compared with propionate alone by a factor 1.5 - 1.7 according to substrates concentrations. Combinations of propionate + pyruvate decrease glucose formation in comparison with propionate alone. Conversion of pyruvate into lactate is enhanced by propionate. Glucose formation from lactate is strongly decreased by 14 micrometers quinolinate but has no significant effect on glucose formation from propionate. Amino-oxyacetate (0.2 micrometers) which decreases gluconeogenesis from lactate produced a slight enhancement of gluconeogenesis from propionate. 10 micrometers n-butylmalonate decreases gluconeogenesis by about 20-30% from three substrates. These observations are consistent with a predominant conversion of propionate into phosphoenolpyruvate intramitochondrially. Butyrate is rapidly metabolized by isolated rabbit liver cells with formation of ketone bodies. It inhibits glucose formation from propionate. 10 micrometers ammonium chloride + 2 micrometers ornithine in presence of propionate give an important ureogenesis and strongly decrease gluconeogenesis from propionate. At low concentration (0.5 micrometers), butyrate partially raises the inhibiting effect of ammonia on gluconeogenesis. This effect is progressively annulled as butyrate concentration rises to 10 micrometers. These observations are consistent with the fact that butyrate modifies the intramitochondrial ratio NADH/NAD and thereby the oxidation of malate formed from propionate.