Biosynthesis of protein in rat testes is known to have a temperature optimum at 32° C; the normal scrotal temperature. The incorporation of palmitic acid into lipids was examined for a temperature dependence. Testicular tissue was incubated for 2 hr with palmitic acid-U-14C at 28, 30, 32, 34 and 36° C. Lipids were extracted and lipid classes separated by thin-layer chromatography. Palmitic acid oxidation was measured. Most lipids incorporated palmitic acid with a positive temperature coefficient. The percentage increases in incorporation of fatty acid over the 8° C range were: free fatty acid (58), triglycerides (51), sterol esters (56), diglycerides (19), monoglycerides (marked decline), sterols (no change), phosphatidyl choline (66), lyso-phosphatidyl choline (36), cephalins (25). The percentage distribution of neutral lipid was free fatty acid (9), triglycerides (4), sterol esters (29), diglycerides (6), monoglycerides (6), sterols (25). The percentage distribution of phospholipid was: phosphatidyl choline (41), lyso-phosphatidyl choline (3), cephalin (44), sphingomyelin (4). Palmitic acid oxidation increased 13% over the 8° C range. It is concluded that lipid synthesis is not as sensitive to temperature change as protein synthesis. Adverse effects of temperature on lipids are probably a consequence of impaired protein metabolism.