Summary.Pyrazon (5‐amino‐4‐chIoro‐2‐phenyl‐3(2H)‐pyridazinonc) labelled with tritium in the phenyl ring, was applied to both the root and shoot of seedlings of red beet(Beta vulgaris L., cultivar Detroit Dark Red I, German millet(Setatra italica J.), and tomato(Lycopersicon esculentumMill., cultivar Heinz 1350). Of these species, red best is tolerant, millet is moderately tolerant, and tomato is susceptible to pyrazon.When the roots were exposed to 1–325 ppm3H‐pyrazon in the nutrient solution, the greatest root absorption and transiocation to the shoot was observed in tomato and the least in red beet, while millet was intermediate.Applications to the first true leaf indicated that pyrazon was absorbed by the foliage of all three but that it was not transported basipetally. The absorption and translocation studies indicated that pyrazon mi transported primarily in the xylem.A radioactive metabolite of3H‐pyrazon was detected in the shoots of red beet, but there was no evidence for its presence in the shoots of the other two species. Since this metabolite is radioactive, its structure must contain at least some altered form of the phenyl ring which contained the tritium in the parent molecule. This differentiates it from 5‐amino‐4‐chloro‐pyridazinone, the previously proposed degradation product of pyrazon, since this compound lacks the phenyl ring.On the bash of these results, it is hypothesized that pyrazon will be phytotoxic if a suffic inn mm entration of the unaltered chemical reaches the site of herbicidal action in the shoot.Migration et métabolisme de la pyrazone dans les esp