Wheat cv. Timgalen and cv. Gabo, sunflower cv. HA 124, and sorghum cv. RS 610 plants were grown at normal atmospheric or an elevated CO2concentration (atmospheric + 500 μmol CO2mol air−1). Enrichment with CO2advanced floral initiation by 1 – 3 days in wheat and sunflower but had no such effect in sorghum. The rate of differentiation of the primordial inflorescence was unaffected in sunflower but was reduced slightly in wheat and significantly in sorghum. Exposing plants to a brief CO2enrichment in series during the vegetative phase showed that the sensitive period for stimulation of floral initiation in wheat commenced within 10 days before the formation of double ridges. The sensitive period for retardation of inflorescence differentiation in sorghum commenced within 6 days before floral initiation. For sunflower the sensitive period was short, with peak stimulation occurring 7 days before floral initiation. It is concluded that the flowering response to CO2enrichment was not due solely to a general growth response; rather, direct morphogenic effects were involved.