Leaf extension rates of young maize plants were measured in the field. Large diurnal fluctuations of air temperature from about 5—35°C caused extension rates to vary from 0·4 mm h−1– 3·6 mm h−1. When the temperature of the shoot apical meristem was kept at 30–34°C, extension rates remained constant, despite diurnal fluctuations of air temperature. Leaf water potentials (ψ) above −8 or −9 bars had little apparent effect on extension rate. It was concluded that ψ gradients within leaves could account for this.