Two methods of obtaining air samples were used over 4-month periods in 1976 and 1977 to quantitate trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) vapors arising from a silt loam. With the first method, only the air from an enclosure covering 0.37 m2 of soil was sampled. The second method used air probes to sample the air above the field. Rainfall and resultant soil moisture were related to trifluralin vaporization from field soil. Soon after trifluralin incorporation (0.84 kg/ha) and, while the soil was relatively dry, there was very little herbicide vaporization. Trifluralin vapors accumulated rapidly during a period in which several centimeters of rain fell, but with continued precipitation the vapor density declined. The influence of rainfall and resultant soil moisture upon vaporization became less apparent as the soil concentration of the herbicides declined. Less than 0.5% of the applied trifluralin was detected as vapors during each 4-month period.