The viscoelastic flow through undulating channels is experimentally studied by using 0.1 and 1.0 wt% aqueous solutions of polyacrylamide (PAA). In the measurement of the pressure loss, it is confirmed that the excess pressure loss occurs for both concentrations of PAA. However, the magnitude of the excess pressure loss for the 1.0 wt % solution of PAA is much smaller than that for the 0.1 wt % of PAA. The Deborah number at the occurrence of the excess pressure loss is within the range of 0.1-0.4 and it depends on the concentration of PAA and channel geometry. The flow pattern changes to an asymmetric one including a small disturbance when the excess pressure loss occurs. Flow unsteadiness observed in the 0.1 wt % solution of PAA is discussed in connection with the excess pressure loss. In the velocity measurement, an asymmetric character of measured profiles for the velocity provides evidence for its three-dimensional character. The variation of the stretch rate on the center line of an undulating channel indicates a significant phase shift beyond the occurrence of the excess pressure loss. (C) 2000 The Society of Rheology. [S0148-6055(00)01001-4].