Protoplasts isolated from red-light-grown maize (Zea mays L.) coleoptiles shrank transiently upon brief exposure (e.g. 30 s) to blue light under background irradiation with red light. The maximal volume reduction (about 4% at a saturating fluence) occurred about 5 min after blue-light stimulation. The response was prevented by the anion-channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid. Red light and far-red light did not induce any comparable response. Protoplasts of different sizes and those isolated from different coleoptile positions showed similar responses. After treatment with a saturating blue-light pulse, the protoplasts became responsive to a second pulse and gained full responsiveness within 5 min, suggesting that the photoreceptor system involves a dark-reversible component. The response to continuous blue light was also found to be transient. The protoplast volume was reduced during about 6 to 9 min of irradiation and returned within the next 30 min to the control level. The response to continuous blue light was saturated at 30 pmol m-2s-1. However, when the fluence rate was enhanced 10-fold after a period of irradiation at 30 pmol m-2s-1, the protoplasts showed another shrinking response. These and other kinetic results indicate that the photoreceptor system undergoes a photosensory adaptation. Crowth in different zones of the coleoptile was inhibited by blue light transiently after pulse stimulation, as well as during continuous stimulation. It was concluded that the observed protoplast shrinking is related to the blue-light-induced inhibition of coleoptile growth.