The rate and extent of a cell's response to an extracellular stimulus is influenced by regulators that act on the intracellular signalling machinery. Although not directly involved in propagating the intracellular signal, regulators control the activity of the proteins that transmit the signals. To understand this aspect of cell signalling, we have studied the pheromone response pathway in the fission yeast Schizosaccharomyces pombe, a relatively simple signalling system in a genetically tractable organism. We demonstrate this approach by investigating the role of Rgs1, a member of the Regulator of G protein Signalling (RGS) family of proteins. The rgs1 gene was identified through the Sz. pombe genome sequencing project (accession number Q09777) and recognized as having similarity to RGS proteins [Tesmer et al. (1997) Cell 89: 251-261], but this is the first report concerning the activity of the protein. Strains lacking rgs1 (Deltargs1) are hypersensitive to pheromone stimulation and unable to conjugate with a mating partner. Inhibition of mating occurs at a relative late stage in the process as Deltargs1 strains exhibit pheromone-dependent transcription and form shmoos. Expression of SST2 (an RGS protein that regulates pheromone signalling in the budding yeast Saccharomyces cerevisiae) overcomes the hypersensitivity of the Deltargs1 strains but fails to rescue their mating defect.