Sugar beets (Beta vulgaris L., var. MSU 126 × 5460) were treated with 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) applied to the soil. The pyrazon was labeled with either 3H in the phenyl ring, or with 14C at the 4 and 5 positions of the pyridazinone ring. The disappearance of pyrazon and the appearance of metabolites in the soil and shoots were examined 1, 2, 4, 6, 8, 12, and 16 weeks after treatment. The total recoverable radioactivity decreased with time, but there was a greater loss of 3H-labeled compounds than 14C-labeled compounds from the soil. Thin-layer chromatography of the soil and shoot extracts revealed the presence of one metabolite in the soil and three metabolites in the plant. By co-chromatography with known pyrazon derivatives and by comparison of the labeling (14C, 3H, or both), the metabolites in the shoot were identified as N-(2-chloro-4-phenyl-3(2H)-pyridazinone)-glucosamine (hereinafter referred to as N-glucosyl pyrazon), 5-amino-4-chloro-3(2H)-pyridazinone (hereinafter referred to as ACP), and ACP-complex. The other moiety in the ACP-complex was not identified. The metabolite in the soil was identified as ACP, and was detectable 2 weeks prior to its appearance in the shoot, suggesting that it was absorbed from the soil.