Two experiments were conducted that confirmed the hypothesis that a dietary bicarbonate supplement will improve eggshell quality in hens at high temperatures as long as feed is consumed during the period of eggshell formation. End-of-lay hens were maintained on continuous light at temperatures of 30 and 35 C. Individual egg weights and shell quality measures for each hen were calculated as a proportion of the initial values determined during an acclimatization period at 25 C. Improvements in shell breaking strength in both experiments were observed as a result of supplementing control diets with 1% sodium bicarbonate (NaHCO3). This response to NaHCO3 was not a reflection of a reduced rate of lay or egg mass output, as these were similar or inferior on the control diets. Similar feed intakes on the control and NaHCO3 diets indicated that the response was not related to differences in calcium intakes. Supplements of zinc methionine and ascorbic acid proved to be inferior to NaHCO3. Improvements in egg weight were associated with the introduction of continuous lighting.