Specific receptors for angiotensin II (AII) were expressed in albino Xenopus laevis oocytes co-injected with poly(A)+ mRNA isolated from rat adrenal cortex and the calcium-specific photoprotein, aequorin. In such oocytes, AII elicited rapid, dose-dependent rises in cytosolic free calcium with light emission responses up to 100-fold above basal levels. Ligand-induced light emission was also observed in oocytes injected with rat brain mRNA and stimulated with acetylcholine and glutamate. These findings demonstrate that mammalian AII receptors expressed in Xenopus oocytes are functionally linked to intracellular Ca2+ mobilization, and indicate the potential value of aequorin-injected oocytes for rapid and specific screening of mRNAs transcribed from expression libraries containing cloned receptor cDNAs.