Rate of fluid secretion by the salivary glands of the blowfly Calliphora is regulated by 5-hydroxytryptamine (5-HT) working in conjunction with cyclic AMP. Although cyclic AMP can exactly mimic the acceleration of fluid secretion produced by 5-HT, the underlying electrical events are completely different. Transepithelial potentials were measured by a liquid paraffin-gap technique which permits continuous potential recordings during rapid changes of the bathing medium. The potential of the lumen of unstimulated glands is + 5 mV with respect to the bathing medium but becomes - 10 to 20 mV after applying 5-HT. After stimulation with cyclic AMP, however, the luminal potential becomes more positive (+30 to 40 mV). A similar effect is obtained with theophylline or when glands are treated with 5-HT in the presence of an impermeant anion such as isethionate. These observations suggest that in addition to stimulating the synthesis of cyclic AMP, 5-HT may also act directly to increase anion movement. Cyclic AMP appcars to stimulate cation transport, which explains the increase in positive potential obtained when this compound (or theophylline) is applied in the absence of 5-HT.