The factors that induce kidney growth in K+-depleted animals are unknown. To determine if the low extracellular fluid K+ concentration could act as a growth stimulus, cultures of monkey kidney epithelial cells from the BSC-1 line were studied in media with a low-K+ concentration. Growth of confluent cultures was accelerated maximally at a K+ concentration of 3.2 mM, whereas concentrations of 2.9 and 3.5 mM were also stimulatory but to a lesser extent. Because growing renal tissue from K+-depleted rats was previously found to exhibit increased uptake of nutrient molecules, evidence for enhanced uptake was sought in BSC-1 cells after exposure to low-K+ medium. The uptake of 10 different nutrient molecules was enhanced in cells exposed to low-K+ medium for 30 s. These observations indicate that a reduced extracellular K+ concentration per se stimulates proliferation of renal epithelial cells in culture and could be one of the factors that mediate kidney growth in K+-depleted animals.