A simple method is described for the evaluation of the extensional viscosity of dilute polymer solutions typically used as drift control adjuvants in agricultural sprays. Extensional viscosity characteristics were determined for a wide range of polymer types and molecular weights, by measurements of the flow of solutions through a packed bed of screens over a range of pressure. The polymer types are classified according to their extensional viscosity-strain rate response characteristics. The effects of molecular weight, concentration and polymer type are discussed. A correlation is demonstrated for all the polymer solutions tested relating the extensional viscosity at strain rates occurring in agricultural spray processes with the droplet size distribution Shear viscosity and polymer concentration also affect the median droplet size, but no correlation exists over all polymers.