The diurnal course of stomatal conductance to water vapour diffusion and leaf temperature of mature, fully expanded top leaves of maize plants (Zea mays L. cv. Pioneer PNR 542) grown under different soil water regimes in a semi-arid region in South Africa, were measured during the flowering stage under typical hot, dry summer conditions. The values of conductance indicated major stomatal opening during the early morning, followed by an increasing reduction in stomatal conductance thereafter until sunset. This pattern in stomatal conductance was found to occur irrespective of soil water status under the specific harsh conditions of this study. Since most of the measuring days were typical clear and hot summer days where climatological variables were very well coupled and fluctuated together, the separate effects of independent variables (vapour saturation deficit, air and leaf temperature, net radiation) on stomatal conductance could not be evaluated adequately. However, both the development of the early morning peak in stomatal conductance and the pattern in its decline thereafter seemed to be strongly influenced by the prevailing air temperature.