Ehrlich ascites tumor cells! loaded with H-labeled arachidonic acid and C-labeled stearic acid for two hours, were washed and transferred to either isotonic or hypotonic media containing BSA to scavenge the labeled fatty acids released from the cells. During the first two minutes of hypo-osmotic exposure the rate of H-labeled arachidonic acid release is 3.3 times higher than that observed at normal osmolality. Cell swelling also causes an increase in the production of C-stearic acid-labeled lysophosphatidylcholine. This indicates that a phospholipase A is activated by cell swelling in the Ehrlich cells. Within the same time frame there is no swelling-induced increase in C-labeled stearic acid release nor in the synthesis of phosphatidyl C-butanol in the presence of C-butanol. Furthermore, U7312, an inhibitor of phospholipase C, does not affect the swelling induced release of C-labeled arachidonic acid. Taken together these results exclude involvement of phospholipase A, C and D in the swelling-induced liberation of arachidonic acid. The swelling-induced release of H-labeled arachidonic acid from Ehrlich cells as well as the volume regulatory response are inhibited after preincubation with GDPßS or with AACOCF, an inhibitor of the 85 kDa, cytosolic phospholipase A. Based on these results we propose that cell swelling activates a phospholipase A - perhaps the cytosolic 85 kDa type - by a partly G-protein coupled process, and that this activation is essential for the subsequent volume regulatory response.