1. Test meals of solutions and suspensions of potassium and sodium salts of a series of saturated fatty acids, from C(2) to C(18), were given to healthy subjects.2. From the volume of these meals recovered after a fixed interval the relative effectiveness of the salts of the acids in slowing gastric emptying was assessed.3. On a molar basis the salts of fatty acids from acetic up to decanoic were relatively ineffective in slowing gastric emptying.4. The salts of fatty acids with 12-18 carbon atoms were much more effective than those with up to 10 carbon atoms. Myristate was the most effective of the salts.5. Buffering the test meals with 33 mM sodium citrate increased the effectiveness of the salts in slowing gastric emptying.