This study is concerned with the responses of internode extension rate in white-light-grown Sinapis alba seedlings to light: the kinetics, nature and organ localization of the photoreceptors and possible transduction chains. Phytochrome status was modified either by means of red, or far-red, light pulses given at the end of the photoperiod, or by supplementing white fluorescent light with different fluence rates of far-red light during the photoperiod. The status of specific blue light-absorbing photoreceptor(s) was modified by supplementing a background of blue-deficient light with different fluence rates of blue light. Low Pfr/P established in the leaves induced a promotion of internode extension rate that persisted for ca 24 h after the plant returned to high Pfr/P. The effect of Pfr/P during dark periods is due mainly to this persistent response. Several fluence-rate-dependent responses were revealed: (a), blue light reaching the leaves modulates the responsiveness of the internode to a reduction of Pfr/P restricted to the internode itself; (b), blue light reaching the leaves inhibits internode extension in subsequent darkness; (c), for a given period in darkness, internode growth rate is higher if Pfr/P is reduced before, rather than at the beginning of darkness, or at the beginning of darkness, rather than later; (d), the transition from light to darkness causes a rapid reduction (lag 10 min) of internode extension rate. A correlation (kinetics and localization) between the extracellular activity of one moderately acidic peroxidase isoform extracted from the internodes, and the effect of Pfr/P on internode extension rate is presented. The activity of another extracellular peroxidase isoform was unaffected by Pfr/P, but showed a rapid increase after wounding. This treatment caused no significant effects on the isoform controlled by Pfr/P. Present findings are discussed in relation to the ecological significance and the physiological basis of the control of internode extension growth by light.