The viscosity of commercially available impression materials used for making edentulous impressions was determined as a function of time and rotational speed. The impression materials studied showed large differences in viscosity and could be ranked into three groups. The values for one group--Plastogum, Ackerman cement (1:1 powder-to-liquid ratio), and light-bodied Permlastic--ranged from 23,800 cp to 57,200 cp. The viscosity values for a second group--Unilastic, syringe Elasticon, Luralite, and Coe-Flo--ranged from 92,200 cp to 107,000 cp. For a third group--Omniflex, Jeltrate, and Ackerman cement (2:1 powder-to-liquid ratio)--the viscosity values ranged from 239,000 cp to 257,000 cp. Light-bodied Permlastic, Ackerman cement (1:1 powder-to-liquid ratio), Jeltrate, and syringe Elasticon functioned as Newtonian liquids immediately after mixing. All other materials were non-Newtonian in behavior. The viscosity of most impression materials increased dramatically with time; the only exceptions were the zinc oxide-eugenol impression materials.