Polydimethylsiloxanes (PDMSs) with a narrow molecular weight distribution and fitted at one chain end with a hydrogenodimethylsilyl or a dimethylvinylsilyl function were prepared by the anionic ring-opening reaction of hexamethylcyclotrisiloxane in a 50 vol% benzene-tetrahydrofuran solution, followed by endcapping with chlorodimethylsilane or chlorodimethylvinylsilane. The experimental conditions were chosen on the basis of preliminary experiments in order to minimize the contribution of reshuffling reactions and it was checked that the possible formation of difunctional species was negligible. The functionalization was quantitative. The functional polymers were characterized by i.r. spectroscopy and by s.e.c. alpha-Hydrogenodimethylsilyl and alpha-dimethylvinylsilyl PDMS were prepared in the molecular mass range between 2500 and 40 000 g mol(-1).