1. In hepatocytes isolated from 24 h-starved rats, urea production from ammonia was stimulated by addition of lactate, in both the presence and the absence of ornithine. The relationship of lactate concentration to the rate of urea synthesis was hyperbolic. 2. Other glucose precursors also stimulated urea production to varying degrees, but none more than lactate. Added oleate and butyrate did not stimulate urea synthesis. 3. Citrulline accumulation was largely dependent on ornithine concentration. As ornithine was increased from 0 to 40 mM, the rate of citrulline accumulation increased hyperbolically, and was half-maximal when ornithine was 8-12 mM. 4. The rate of citrulline accumulation was independent of the presence of lactate, but with pyruvate the rate increased. 5. The rate of urea production continued to increase as ornithine was varied from 0 to 40 mM. 6. It was concluded that intermediates provided by both ornithine and lactate are limiting for urea production from ammonia in isolated liver cells. It was suggested that the stimulatory effect of lactate lies in increased availability of cytosolic aspartate for condensation with citrulline.

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