The actual transpiration of corn (Zea mays L.) was determined by hydraulic lysimeters, by the inflow-outflow method and by use of a neutrone probe. Two irrigation treatments were used; wet and dry. Under the wet treatment, 740, 858, and 743 mm of irrigation water was used during the seasons 1968, 1969, and 1970 respectively. Under the dry treatment, amount of irrigation water used was 560, 627, and 565 mm during the seasons 1968, 1969 and 1970, respectively. Ratios of actual transpiration (Et) to Penman's estimate of potential evaporation (Eo), evaporation from a Class-B pan (EpB), global radiation (Rg) and net radiation (Rn) were calculated. Values of Et measured by the different methods are in good agreement. Seasonal averages of Et, were 6·2 and 4·9 mm/day under wet and dry treatments, respectively. The Et/Eo, Et/EpB, Et/Rg and Et/Rn ratios increased with leaf area index (L.A.I.) and reached their maximum values when L.A.I, reached its maximum value of 1·03 under wet and 0·92 under dry treatment. The empirical coefficient (f = Et/Eo) under wet treatments was found to be higher than that given by Penman for short grasses and about the same as that obtained by Boumans et al. (1963) for golden-gram (Phaseolus aureus) in Iraq for the months August and September. The empirical coefficient Kb in the Blaney–Criddle formula was greater than the value given by Blaney and Criddle for an arid climate, and less than the value obtained by Boumans et al. for golden-gram in Iraq. It was further found that water use efficiency for grain production was greater under dry than under wet treatments.