Minimum doses of metribuzin that provide satisfactory control (>90%) of redroot pigweed (Amaranthus retroflexus L.) and wild mustard (Sinapis arvensis L.), 2 troublesome weeds of potato crops planted in spring and autumn, respectively, were assessed. Greenhouse experiments were conducted using 100%, 80%, 60%, 40%, 20%, and 0% of the registered dose (525 g a.i. ha(-1)) of metribuzin applied to the weeds grown in plastic pots. Dry weight data from the experiments were subjected to nonlinear regression analysis and the effective doses of metribuzin that caused a 90% reduction in dry weight (ED(90)) were estimated. Metribuzin at doses of 240.8 and 293.2 g a.i. ha(-1) resulted in a 90% reduction in the dry weight of A. retroflexus when applied at the BBCH 12-15 and BBCH 16-19 growth stages, respectively. Metribuzin at doses of 183.7 and 256.2 g a.i. ha(-1) provided 90% control of S. arvensis at the BBCH 12-15 and BBCH 16-19 growth stages.