1. Mature laying hens were subjected to ambient temperatures sufficient to maintain body temperature of 43 °C for periods of 6 to 7 h during the day (eight periods) or the night (five periods). This did not reduce total daily food consumption.

2. The effect of heat stress during the day was mostly on egg‐shell quality, being adverse and significant. During the night heat stress caused a significant decrease in egg production, its effect during the day was less marked in this respect.

3. Heat stress thus seems mostly to affect the early stages of shell formation, while its effect on egg production seems to depend on the time at which it occurs in relation to ovulation.

4. These results suggest that egg production is directly affected by heat stress; prolonged heat stress probably also acts indirectly, by suppression of food intake.