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The hydrocyanic acid potential (HCN-p) of forage sorghum [Sorghum bicolor (L.) Moench] and sudangrass [S. sudanense (Piper) Stapf] seedlings may be estimated by a spectrophotometric procedure that is simple and rapid. As assayed by this procedure, seedlings of a number of lines appeared to have increased HCN-p when grown under increased levels of radiation. In investigations of this apparent increase, 7-day seedlings of a number of forage sorghum and sudangrass lines were grown under photosynthetically active radiation levels of about 180μE • m-2 • sec-1 (PAR-1) or 400μE • m-2 • sec-1 (PAR-2). Extracts of first leaves were diluted in 0.1 NNaOH and scanned from 400 to 250 nm. These scans revealed the occurrence, especially in leaf extracts from low HCN-p seedlings grown under PAR-2, of substances that interfered with the 330 nm absorption maximum of p-hydroxybenzaldehyde (p-HB), on which the spectrophotometric assay is based. Fractionation of the aqueous extracts with ether effectively separated p-HB from the interfering compounds. Also, p-HB added to the aqueous extracts was effectively recovered in the ether phase following fractionation. It was concluded that seedlings to be assayed by the spectrophotometric method should be grown under radiation levels no higher than 200μE • m-2 • sec-1, and it was recommended that spectra be scanned and/or ether extraction be used for any entries assaying less than 250 ppm HCN-p by this method.