The seasonal variation in the oil content of cod livers is shown to affect the liberation of free fatty acids (FFA) when the "sun-rotted" method for producing cod-oil is used under laboratory conditions. Since lean livers produce an oil with a high FFA content, oil of this type would most likely be produced in April–June after the fish have spawned. An antibiotic, chlortetracycline, was found to have little effect on the liberation of FFA and cannot be used as a liver preservative without an enzyme inhibitor added. Isopropanol at suitable concentrations inhibits both the liberation of oil from the livers and the formation of FFA in the oil.