1. A rapid-reaction apparatus was used to measure the rate at which cells containing sickle-cell haemoglobin (HbS) undergo morphological changes as a result of very rapid deoxygenation. The events occurring under these circumstances were found to take place in two stages. 2. The first consisted of a very rapid gelation of the cellular haemoglobin producing bizarre-shaped erythrocytes. The rate of this process was determined by the rate of cellular deoxygenation. Starting at 100% HbO2 there was, however, a delay of about 30 ms corresponding to the time of formation of the minimum amount of Hb to allow sickling to occur. 3. The second stage was very much slower and resulted in the cells gradually acquiring the more usual holly-leaf and sickle-forms. 4. It was also possible to show with this apparatus that formalin is an extremely rapid fixative. Cells mixed with this solution became fixed within 14 ms.