1. The incorporation of l-[1-(14)C]leucine into the proteins of liver slices and into the serum albumin and lipoproteins transported by these slices was investigated. 2. Transport rates were found to be dependent on the K(+) content of the slices. 3. The effect of K(+) on transport of serum albumin and of serum lipoprotein can be separated from any effect on synthesis by altering K(+) concentrations after inhibition of protein synthesis by cycloheximide or puromycin. 4. The effect of low K(+) concentrations is reversible. 5. There is linear relationship between the K(+) content of the slices and the transport of protein. A simple method is described for maintaining various steady concentrations of K(+) in the liver slices. 6. K(+) may be replaced by Rb(+). Cs(+) is partly effective, but NH(4) (+) and Li(+) are no more effective than Na(+). 7. We found evidence that K(+) content rather than the flux rates of K(+) or Na(+) is important in this effect. 8. These results are probably important in ethionine and carbon tetrachloride poisoning in the rat, and may be significant in liver transplantation.