Abstract.Fluence‐rate response curves were determined for the inhibition of hypocotyl growth in 54‐h‐old dark‐grownSinapis albaL. seedlings by continuous or hourly 5 min far‐red light irradiation (24 h). Just as in red light (Heim&Schäfer, 1982), a fluence‐rate dependence was observed for both kinds of irradiations, even if only 35% of the continuous light effect could be substituted for by hourly far‐red pulses. The same total fluence was used for the two different light regimes. Measurements of Pfr and Pfr/Ptot showed a strong fluence‐rate dependence under continuous light which only partially paralleled the fluence‐rate response curves for the inhibition of the hypocotyl growth. It was concluded that neither spectrophotometrically determined levels of Pfr nor Pfr/Ptot can be the only light‐dependent factor controlling hypocotyl lengthening under co