Wheat seedlings (Truicum aestivum L. cv. Starke II, Weibull) grown in a solution of the herbicide SAN 9789 axe deficient in chlorophyll and carotenoids. Such seedlings were used in order to isolate a blue light response of stomata, not mediated by photosynthesis. When illuminated with blue light (430-505 nm) SAN-grown seedlings showed a considerable transpiration response, whereas red light (>590 nm) gave no response whatsoever with the intensities used. The transpiration was measured with hygrometers in an open air-flow system. Furthermore, blue light had a superior effect, relative to red, in causing a transpiration response in untreated green seedlings. The transpiration level after two hours of illumination was higher in blue than in red light, although the blue light had its major effect immediately after the light was switched on. The difference between the effects of blue and red light was most pronounced at low light intensities. This, together with the high sensitivity of SAN-grown seedlings to low ntensity blue light, led to the conclusion that blue light is of special importance at low quantum fluxes. Because of the very low carotenoid content in the SAN-grown seedlings, the role of the carotenoids as photoreceptors for the blue light response of stomata was excluded.