Abstract: The helical flow of power-law drilling muds in the drill string was studied both theoretically and experimentally. The analytical expressions of the apparent viscosity distribution, the velocity distribution and the flow ratefor the helical laminar flow were derived. The equation giving the pressure drop, the expressions of the stability parameter K, characterizing the transition from laminar to turbulent flow, and its maximum value K_max were established. Experimental studies with aqueous solutions of CMC and PAM showed that the helical laminar flow of the power law fluid in the pipes is transformed into turbulent flow when K_max=400.