The effects of cyclic adenosine 3': 5'-monophosphate (cAMP) on the growth of Arena coleoptile segments over 4 to 10 hours were monitored with a position sensing transducer. At pH 6, cAMP (0.1 mM with and without 2.5 mM glucose; or 2 mM alone) or dibutyryl cAMP (0.1 mM) was added at the beginning of the experiment, or after about 1 hour or after about 6 or 7 hours. Under all conditions tested, cAMP compounds had little or no effect on coleoptile segment elongation. Inasmuch as cAMP does not duplicate the rapid and vigorous elongation obtained with 2 gM auxin, the hypothesis that cAMP is a mediator of auxin activity is not supported by experimental evidence in this system. This conclusion is dependent upon the assumption that the cAMP compounds penetrated the tissue.