Abstract.The cellular divisions in mass cultures of the ciliate Tetrahymena pyriformis were synchronized through heat shocks. Changes in the protein metabolism during the heat treatment and the subsequent first synchronous division have been analysed by aid of one‐dimensional paper chromatography of the free amino acids and nitrogen estimation of the soluble and insoluble proteins. Both the free amino acids and the total protein content increased about 3 times during shock treatment. Most individual free amino acids varied in amount in the same manner. At the higher temperature the protein synthesis showed to be blocked and as a result the synthetic activity proceeded in distinct waves corresponding to the individual shocks. The synthesis of soluble proteins showed a lag in time compared to the synthesis of the insoluble proteins. The protein synthesis was found to stop before division.