1. The nature of the cyclic AMP-receptor interaction was analysed by testing a range of cyclic nucleotides on the isolated salivary glands of adult blowflies.
2. All compounds containing modifications in the region of ribose or the phosphate ring were inactive. One compound, adenosine 3',5'-phosphorothioate, appeared to compete with cyclic AMP.
3. A number of nucleotides with alterations restricted to the base region of the molecule could stimulate secretion equally as well as cyclic AMP.
4. These observations indicate that during the action of cyclic AMP the phosphate ring and ribose sugar are critical whereas the adenine ring plays a relatively unspecific role.