Absorption and translocation in both leaves and roots is less under low temperatures. Seasonal response of wheat to 2,4-D is conditioned by the stage of growth as well as environment. The response of wheat and downy bromegrass to triazine herbicides is seasonal, whereas the response to carbamate herbicides appears to be influenced by local environmental conditions at time of spraying. Physiological condition of fiddleneck is more important than age, i.e., plants in the rosette stage were always far more sensitive than bolting plants of the same age. Sensitivity to 2,4-D was greater at 26 C than at 10 C or 5 C at all stages of growth. Applications of certain metabolites, especially thiamin, increased the sensitivity of fiddleneck to 2,4-D at low temperatures.