Uridine and uracil nucleotides are involved in the regulation of various cell functions. Here, Roland Seifert and Günter Schultz review the evidence that, rather than by binding to purinoceptors, pyrimidine nucleotides exert their effects by binding to distinct pyrimidinoceptors, which are coupled to pertussis toxin-sensitive G proteins in human phagocytes. However, many questions remain to be answered: no antagonists for these pyrimidinoceptors are available, and binding studies have not been carried out; the receptor proteins and subtypes have not been characterized; and little is known about the G proteins and effector systems involved, or the regulation of storage and release of pyrimidine nucleotides.