Rheological properties of three immiscible polyisoprene (PI)/polydimethylsiloxane (PDMS) blends (PI:PDMS = 1:9 by weight) having viscosity ratios eta(d)/eta(m) = 0.155, 0.826, and 4.02, where eta(d) and eta(m) are 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 rate (gamma) over dot (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 with eta(d)/eta(m) = 0.155 and 0.826, stresses were proportional to (gamma) over dot and the droplet sizes were well regulated by the flow and inversely proportional to (gamma) over dot for (gamma) over dot greater than or equal to 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. Acta 29, 204-214 (1990)]. For the eta(d)/eta(m) = 4.02 blend, however, the droplet sizes were insensitive to (gamma) over dot, resulting in worse agreement between the experimental and theoretical results than for the other blends.