Endometrium from guinea-pigs on Days 7 and 15 of the oestrous cycle (days of low and high endometrial prostaglandin F2 alpha production, respectively) was maintained in tissue culture for periods up to 24 h (uptake experiments) or 48 h (release experiments). Tritiated arachidonic acid (3H-AA) was incorporated into endometrial phospholipids and neutral lipids in a time-dependent manner. After 24 h of culture, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were the major phospholipids, and triglyceride (TG) was the major neutral lipid which had incorporated 3H-AA. PC, PE and phosphatidylserine/phosphatidylinositol (PS/PI) incorporated significantly more 3H-AA on Day 15 than on Day 7. TG also incorporated more 3H-AA on Day 15 than on Day 7, but the increase was not statistically significant. Tritiated oleic acid (3H-OA) was incorporated into endometrial phospholipids and neutral lipids in a time-dependent manner. No increase in uptake of 3H-OA occurred on Day 15 compared to Day 7. There appears to be a specific stimulation of the mechanisms involved in the uptake of arachidonic acid into guinea-pig endometrium (particularly into the phospholipids) at the end of the oestrous cycle. There was little apparent release of 3H-AA from any endometrial lipid class, except diglyceride (DG) and monoglyceride (MG), on Day 7. In contrast, there was an apparent 50 to 80% decrease in the 3H-AA content of several endometrial lipid classes, particularly PC, PE and TG, on Day 15. Overall, the uptake and release studies suggest that PC, PE and possibly TG form the source of free arachidonic acid for PGF2 alpha synthesis by the guinea-pig endometrium.