We studied the responses to hypertonicity of cultured endothelial cells from swine pulmonary arteries. In 0.5 osmol/kgH₂O medium, initial cell shrinkage was followed by a regulatory volume increase (RVI), complete after 1 h, concomitant with an increase in cellular K⁺content. Then the activity of amino acid transport System A increased, accompanied by an accumulation of ninhydrin-positive solutes (NPS), reaching a peak at ∼6 h. The subsequent decline in System A activity was paralleled by an induction of the betaine-GABA transporter (BGT-1), detected as increases of BGT-1 mRNA and of transport activity, which peaked at ∼24 h. Inhibitors of transcription or translation prevented induction of both transport activities. The increased expression of BGT-1, which involved activation of “tonicity-responsive enhancer,” was inhibited by 5 mM extracellular betaine. Cellular K⁺concentration gradually declined after the accumulation of NPS and during the induction of BGT-1. This very effective adaptation to hypertonicity suggests it has a physiological role.