The flow of power law fluids for isothermal, laminar flow in concentric annuli was investigated experimentally and numerically. Similar to the results of Russel & Christiansen (1974, Indust. Engng Chem. Process Des. Develop. 13 , 391–398) it was observed that plots of the natural logarithm of a reduced shear stress at the outer wall of the annulus vs the natural logarithm of a reduced pseudo-shear rate were identical to those for tube flow. However, in contrast to the findings of Russel & Christiansen (1974), the y intercepts and the slopes of these plots were found to be equal to In K′ and n, respectively. Frederickson & Bird's (1958, Indust. Engng Chem. 50 , 347–352) solution was found to be in perfect aggreement with the numerical solution in every case. The Hanks & Larsen (1979, Indust. Engng Chem. Fundamentals, 18 , 33–35) equation also agreed very well with the numerical solution. The Russel & Christiansen (1974, Indust. Engng Chem. Process Des. Develop., 13 , 391–398) method gave an absolute mean error of 3.6% against the numerical results for pressure drop.