Rheological properties of three immiscible polyisoprene (PI)/polydimethylsiloxane (PDMS) blends (PI:PDMS=1:9by weight) having viscosity ratiosηd/ηm=0.155,0.826, and 4.02, whereηdandηmare the viscosities of the droplet and matrix, were investigated by directly measuring the droplet size distributions with a video microscope. Steady state measurements were performed by sequentially stepping up the shear rateγ̇(step-up) and step decrease (step-down) of the shear rate. Dynamic frequency sweep measurements were also performed immediately following cessation of steady shear flow. In the step-up measurements for blends withηd/ηm=0.155and 0.826, stresses were proportional toγ̇and the droplet sizes were well regulated by the flow and inversely proportional toγ̇forγ̇⩾0.553 s−1,in good agreement with the Doi–Ohta model [J. Chem. Phys.95, 1242–1248 (1991)]. The results of step-down measurements were consistent with dynamic moduli measurements, because the droplet sizes were almost unchanged during these measurements. The results are in accord with Palierne’s model [Rheol. Acta29, 204–214 (1990)]. For theηd/ηm=4.02blend, however, the droplet sizes were insensitive toγ̇,resulting in worse agreement between the experimental and theoretical results than for the other blends.