A fluorometric method to determine the kinetics of the uptake and metabolism of a p-halohydroxybenzonitrile herbicide has been developed using variable chlorophyllafluorescence. Bromoxynil inhibits photosynthetic electron transport and results in large variable chlorophyllafluorescence yields. Lethal and sub-lethal doses of bromoxynil were applied to two maize genotypes. Measurement of variable chlorophyllafluorescence over several hours revealed the uptake and detoxification of bromoxynil by the appearance and disappearance of inhibited fluorescence transients. A difference in bromoxynil susceptibility was observed between inbreds B73 and OH43 in these preliminary studies. Also observed was a difference in susceptibility between the oldest and second oldest leaves of these maize seedlings. Variable chlorophyllafluorescence has the potential to be a rapid, non-destructive analytical method for observing the biochemical metabolism of herbicides in plants.