The short-term response of etiolated Sinapis alba to blue irradiation was compared with red and far-red responses. The rate of hypocotyl elongation was measured with displacement transducers and recorded with a microcomputer based data acquisition and control system. Blue light (11 W m-1) inhibited the growth rate after a lag of only 40-60 s, while red (8 W m-1) and far-red (7 W m-1) light had a much slower and weaker effect. These results show that a specific blue-light photoreceptor controls hypocotyl elongation in Sinapis. A possible accessory role for phytochrome in the rapid blue-light response was tested by dichromatic irradiations. The rapid blue response was unaffected by simultaneous far-red irradiation of equal quantum flux, indicating that phytochrome cycling is not involved in the rapid growth suppression by blue light. These results show that elongation of Sinapis hypocotyls is regulated by two distinct photoreceptors.