Uridine triphosphate (UTP) evoked inhibition of adrenaline-evoked cAMP accumulation in cultured equine epithelial cells (EC50, 1á8 ± 0á2 µM) and this effect was mimicked by 5-Br-UTP (EC50, 6á6 ± 1á8 µM) and uridine diphosphate (UDP; EC50, 96 ± 26 µM). This inhibitory action of UTP was abolished by pre-treating cells with pertussis toxin (10 ng ml-1, 24 h). UTP (EC50, 2á3 ± 0á3 µM) and 5-Br-UTP (EC50, 29á4 ± 9á4 µM) also increased intracellular free calcium ([Ca2+]i) whilst UDP did not; the two effects are thus differentially sensitive to these pyrimidine nucleotides. ATP evoked cAMP accumulation in control cells and this response was unaffected by pertussis toxin. There is, therefore, no indication that ATP activates the pertussis toxin-sensitive inhibitory pathway. The UTP-evoked inhibition of cAMP accumulation was abolished by isobutylmethylxanthine (IBMX, 5 mM) and so the negative control over cAMP levels appears to be mediated by receptors that are selectively activated by pyrimidine nucleotides and permit control over phosphodiesterase activity.