1. Diets containing supplements of selenium (10 mg/kg as sodium selenite), copper (500 mg/kg as copper (II) sulphate) and sodium nitroprusside (SNP 300 mg/kg) as a source of cyanide, were fed singly and in all possible combinations to chicks from 14 to 35 d of age. 2. Both copper and SNP individually alleviated the growth depression caused by excess selenium but interacted adversely with one another. The effect of SNP was to decrease liver selenium but copper increased it. Despite this contrast it is suggested that both achieve their beneficial effects through influences on the fraction of liver selenium that can be reduced to volatile forms by hydrochloric acid and zinc. 3. In a second experiment the effects of diets containing additional selenium (2 mg/kg as sodium selenite or selenomethionine) with or without additional copper (100 mg/kg) or SNP (100 mg/kg) on selenium incorporation into eggs were compared. 4. SNP reduced incorporation from selenite whereas copper had no effect. However, copper reduced the incorporation from selenomethionine into protein fractions of egg white as much as SNP.