The effect of cell swelling, induced by a hyposmotic challenge, upon the efflux of radiolabelled taurine and iodide from term human placental tissue explants has been studied. A hyposmotic shock markedly increased the unidirectional efflux of taurine in a manner which was fully reversible. It took approximately 6-8 min for taurine efflux to reach a maximum response following a hyposmotic challenge whereas it took about 16 min for taurine efflux to return to a basal level. This suggests that factors other than, or in addition to, membrane stretch are involved in the signal transduction process. A prolonged hyposmotic challenge appeared to inactivate volume-sensitive taurine release. Volume-activated taurine efflux was dependent upon the extent of cell swelling: the effect was greatest when the decrease in the osmolality was greater than 27 per cent. Cell swelling-induced taurine efflux was markedly inhibited by diiodosalicylate and to a lesser extent by bumetanide and quinine. A hyposmotic challenge also increased the efflux of iodide via a pathway which was sensitive to niflumic acid. This is consistent with the presence of volume-activated Cl(-)channels. Volume-activated amino acid and anion (Cl(-)) efflux may act in parallel with K(+)efflux to regulate placental cell volume following swelling.