The specific radioactivities of individual molecular species of muscle phosphatidylcholine and phosphatidylethanolamine have been measured by a combination of argentation thin-layer chromatography and countercurrent distribution. 2. The specific radioactivities of individual molecular species of muscle phosphoglycerides have been determined 3 h after intraperitoneal injection of ortho[32 P] phosphate. Under these conditions the specific radioactivities of the species present in rat muscle were found to be measures of the relative turnover times of these molecules. 3. The specificity radioactivity of phosphatidylcholine was approx. three times that of phosphatidylethanolamine. The 1-palmitoyl-2-oleoyl and 1-oleoyl-2-linoleoyl phosphatidylcholines had the fastest turnover and the 1-palmitoyl-2-arachidonoyl the slowest. Of the phosphatidylethanolamines, the linoleoyl and the docosahexaenoyl species showed the fastest turnover and 1-stearoyl-2-arachidonoyl the slowest. 4. The results indicate that phosphoglycerides in muscle turn over more slowly and more evenly than do liver phosphoglycerides.
