Wild-type fission yeast, growing in minimal medium, actively synthesizes thiamine and maintains an internal concentration which we have measured to be around 10 pmoles/10(7) cells. If thiamine is added to such cultures it is rapidly sequestered by the cell, and if added in excess (20 microM) the internal concentration of thiamine rises almost 1000-fold to a maximum of around 9000 pmoles/10(7) cells before the transport mechanism is shut down. The kinetics of decay of intracellular thiamine to the basal level are consistent with simple dilution as the cell mass doubles. In parallel with this analysis, we have studied the transcriptional activity of the thiamine-sensitive gene nmt1 as a function of intracellular thiamine concentration. Transcription of this gene is rapidly repressed as the internal thiamine concentration rises and is only reactivated as the concentration falls to below about 50 pmoles/10(7) cells.