The effect of temperature and soil moisture content on the toxicity of soil-applied 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) to sugar beets (Beta vulgaris L. ‘U.S. H-8’) was studied under controlled environmental conditions. High temperatures during or after germination increased the susceptibility of sugar beets to pyrazon while variations in soil moisture content did not have a significant effect. Sugar beet seeds absorbed three times more pyrazon at 35 C than at 18.3 C. During imbibition more than 90% of the pyrazon taken up by sugar beet fruits was concentrated in the pericarps surrounding the seeds. Furthermore, the herbicide which had been accumulated in the pericarp during imbibition did not move into the tissues of the developing seedling during or after germination. Comparative studies showed that there was a lag period in absorption of pyrazon by sugar beet seeds enclosed within their pericarps. This lag period did not occur in sugar beet seeds from which the pericarps had been removed, or in seeds of common lambsquarters (Chenopodium album L.). It is concluded, therefore, that the pericarp contributes to a physical mechanism of selectivity which enables sugar beets to avoid great accumulation of pyrazon when the mechanism of biochemical inactivation of the herbicide is not yet operative.