The effect of heat shock on protein synthesis in the Drosophila melanogaster KC 161 tissue culture cell line was examined with a view to investigating the mechanism underlying the acute reduction in normal cellular protein synthesis typical of heat‐shocked Drosophila cells. However, at 36–37°C, the optimum temperature for induction of the 70‐kDa heat‐shock protein, this cell line did not show such a response. The synthesis of a very limited number of proteins was abruptly turned off following heat shock in the presence or absence of actinomycin, but the rate of synthesis of the majority of normal cellular proteins declined slowly over a three‐hour period. Incubation of heat‐shocked cells in hypertonic media increased the relative proportion of protein synthesis directed towards heat‐shock proteins (as opposed to normal cellular proteins). Incubation with low concentrations of cycloheximide had the converse effect and resulted in a preferential increase in the size of polysomes translating normal cellular mRNAs, greater than the increase in size of polysomes synthesising heat‐shock proteins. Heat shock also resulted in some mRNAs being almost completely displaced from polysomes into the postribosomal supernatant. These observations suggest that competition between normal cellular mRNAs and increasing amounts of heat‐shock mRNAs with a higher affinity for the translation machinery was the main explanation for the gradual reduction in the synthesis of normal cellular proteins, although a slight reduction in overall translation initiation rates cannot be excluded as a subsidiary cause. The results demonstrate that the acute reduction in normal cellular protein synthesis seen in other Drosophila cell lines is not an integral and necessary feature of the heat‐shock response in this organism, which makes it unlikely that the mechanism of this acute shut‐off is intimately connected with the mechanism of induction of heat‐shock mRNAs.