Cyclic AMP is a ubiquitous second messenger that transduces signals from a variety of cell surface receptors to regulate diverse cellular functions, including secretion, metabolism and gene transcription. In pancreatic β-cells, cAMP potentiates Ca²⁺-dependent exocytosis^,, and mediates the stimulation of insulin release exerted by the hormones glucagon and glucagon-like peptide-1 (GLP-1) (refs ). Whereas Ca²⁺ signals have been extensively characterized and shown to involve oscillations important for the temporal control of insulin secretion^,,, the kinetics of receptor-triggered cAMP signals is unknown. Here we introduce a new ratiometric evanescent-wave-microscopy approach to measure cAMP concentration beneath the plasma membrane, and show that insulin-secreting β-cells respond to glucagon and GLP-1 with marked cAMP oscillations. Simultaneous measurements of intracellular Ca²⁺ concentration revealed that the two messengers are interlinked and reinforce each other. Moreover, cAMP oscillations are capable of inducing rapid on–off Ca²⁺ responses, but only sustained elevation of cAMP concentration induces nuclear translocation of the catalytic subunit of the cAMP-dependent protein kinase. Our results establish a new signalling mode for cAMP and indicate that temporal encoding of cAMP signals might constitute a basis for differential regulation of downstream cellular targets.