Abstract Addition of 10 m M Ba to suspensions of single-cell embryos of the echnioderm Lytechinus results in a depolarization of membrane potential, an increase in membrane resistance, and a reduction of the unidirectional efflux rate coefficient for potassium. All these effects are consistent with a decrease in membrane permeability to potassium ion ( P K). Determination of P K in the presence of Ba indicates a 3- to 5-fold decrease in its magnitude as compared to Ba free cells. Ba at concentrations of 0.5 m M or less has no significant effect on P K. Its effect is maximal at approximately 10 m M, and it is completely reversible upon Ba removal. Eggs fertilized and maintained in 10 m M Ba do not undergo the 3- to 4-fold increase in P K which follows fertilization but rather, they maintain values for P K equivalent to or less than those found in the unfertilized egg. This allows one to evaluate the relationship of the normal increase in P K to the increase in the exchangeability of intracellular K and the onset of protein synthesis, both of which occur shortly after fertilization. The results indicate that the onset of protein synthesis and the increased exchange of intracellular K are events which are independent of the increase in P K. Neither is significantly altered by Ba suppression of the P K increase. The results also indicate that active K transport and amino acid transport are unaffected by maintenance of a low P K. Furthermore, inhibition of protein synthesis (greater than 95% with 10 −4 M emetine) does not prevent the increased exchange of intracellular K or the increase in P K. Active K transport and amino acid transport are also unaffected.