A study of the kinetics of chlorophyll (Chl) synthesis in cotyledons of etiolated cucumber seedlings (Cucumis sativus L. cv. Delilah) treated with 5×10‐5M‐ben‐zyladenine (BA) showed that cytokinin, like a red light pulse, could inhibit as well as promote pigment accumulation depending on the length of the dark period following induction. Spraying intact, dark‐grown seedlings with BA, 24 h prior to white light exposure, eliminated the lag phase in Chl synthesis, while treatment with hormone 72 h before greening not only delayed the onset of synthesis, but it also reduced the amount of Chl accumulated after 24 h continuous white light. Impairment of Chl formation was correlated with inhibited regeneration of protochlorophyll and delayed appearance of the light harvesting Chlalbpolypeptide. Application of σ‐aminolevulinic acid (15 mM) 2hbefore white light exposure shortened the lag phase in Chl synthesis in control as well as in inhibited cotyledons, but the adverse effect of the red light and BA treatments on long‐term Chl accumulation (24 h) was not reversed. Application of glutamate did not stimulate Chl production. Simultaneous treatment with hormone and red light 72 h before greening enhanced their separate inhibitory effects on Chl synthesis, but when given together 24 h prior to white light, their promotive effects on pigment accumulation were