Limitation of nutrients allows yeast cells to arrest proliferation at G1 phase of the cell cycle and to enter the so-called stationary phase. We show here another pathway for cytostasis, which is associated with extracellular accumulation of bicarbonate and the resulting alkalisation of medium during the proliferation of cells respiring acetate. Alkalisation of medium by addition of bicarbonate or alkaline buffers ceased proliferation at G1 phase of logarithmically growing cells and caused a severe drop in G1-cyclin (CLN1 and CLN2) mRNAs. The arrested cells were heat-shock resistant, suggesting that the cells entered the stationary phase. Cells confluently grown on acetate re-entered into the cell cycle after acidification of the culture medium. These results indicate that external alkalisation is a primary cause of the cytostasis. The alkali-induced G1 arrest was shown to be cyclic AMP (cAMP)-independent using mutant cells which lack a functional Ras/cAMP signaling pathway. Alkalisation of medium also stimulated meiosis and sporulation in rich acetate medium, confirming our previous proposal that environmental alkalisation but not nitrogen limitation is a key condition for entry into meiosis and sporulation.