Extracellular adenine nucleotides have long been known to mediate pharmacological responses via several subtypes of P-2 purinoceptors. More recently, however, similar responses to uracil nucleotides have been demonstrated. Here, Didier Communi and Jean-Marie Boeynaems discuss the evidence to suggest that distinct pyrimidinoceptors do indeed exist, although they do not appear to constitute a separate receptor family of their own.