Self-diffusion and spin-spin relaxation measurements have been performed on a series of blends of narrow-fraction linear and cyclic polydimethylsiloxane polymers in the melt by pulsed nuclear magnetic resonance techniques. The results obtained are compared to the viscosity behaviour on the same samples and discussed in terms of the reptation and Rouse theories commonly invoked for the dynamics of single-component systems. The dynamics of the blend samples are shown to deviate from the expected behaviour above a critical ring size which can be understood through a mechanism based on ring threading.