The dielectric cu-relaxation of a series of narrow-molecular-weight fractions of cyclic and linear polydimethysiloxanes (PDMSs) has been investigated as a function of frequency in the temperature range 140.5-157.5 K. The temperature of the alpha-transition decreases with increasing chain length for cyclic PDMS and increases with increasing chain length for linear PDMS, in agreement with the previously reported differential scanning calorimetry (d.s.c.) glass transition. The cooperativity of the relaxation process, according to the Kohlrausch-Williams-Watts analysis, is greater in linear PDMS. The activation energy and molecular dipole moment increase with increasing chain length for both cyclic and linear molecular types. Experimental values of the dipole moment ratio are in good agreement with those measured by static dielectric techniques and with the theoretical long-chain value.