Diurnal rates of net CO2 assimilation (P-N) and soil respiration (R-h) in the summer were measured in six Mediterranean ecosystems through an automatic continuous monitoring system. The six sites include a typical evergreen Pinus pinea L. forest with two co-occurring sclerophyllous shrubs (Pistacia terebinthus L. and Phillyrea latifiolia L.), citrus (Citrus Union L.), corn (Zea mays L.), cotton (Gossypium hirsutum L.), soybean (Glycine max L.) and vineyard (Vitis vinifera L.). All six sites exhibited similar behaviour in that low soil water availability and high evaporative demand not only depressed P-N and R-h, rates, but also changed the diurnal time course of their peak rates. Except for P. terebinthus and corn, P-N rates of all the species peaked in the morning (7:30-9:30), and R-h, rates in all the sites were higher during the night than during the day (p < 0.05). Mean rates of net ecosystem emission (NEE) Of CO2 to the atmosphere were highest (-23.9 mu mol CO2 m(-2) s(-1)) in the corn site and lowest (-0.6 mu mol CO2 m(-2) s(-1)) in the vineyard site. On average, all six ecosystems were a net source of CO2 to the atmosphere, due to R-h effluxes exceeding P-N rates. Net CO2 assimilation and R-h explained 25% (for corn) to 87% (for citrus) of the diurnal fluctuations of the atmospheric CO2 concentration. Multiple linear regression (MLR) models accounted for 42% (P. pinea) to 95% (cotton) of diurnal variations in P-N rates and 30% (forest) to 92% (citrus) of diurnal variations in R-h rates (p <= 0.001). The dependence of diurnal P-N and R-h rates on water vapour pressure deficit and soil water deficit for dry and hot summer days appeared to be major and needs to be re-examined for biogeochemical models of climate change effects on CO2 dynamics of Mediterranean ecosystems.