A regression model with which the leaf water potential (LWP) of wheat can be estimated from soil water content and potential evapotranspiration was developed from data generated during 1986. This model was used during the 1988 wheat season to decide when to relieve crop water stress by irrigation. Seven values of LWP, viz.−1.2;−1.4;−1.6;−1.8;−2.0;−2.2 and−2.4 MPa were considered as possible threshold values for irrigation. The soil water content, determined twice weekly, was used to estimate actual evapotranspiration. Midday LWP was determined on selected cloudless days and found to be most suitable as an irrigation timing criterion for several reasons. Firstly, the ratio evapotranspiration/pan evaporation was at least as well related to LWP as to soil water status. Secondly, strong relationships between grain yield, seasonal evapotranspiration, peak evapotranspiration rate and threshold LWP exist, and finally, indications are that yields near the upper limit of the evapotranspiration-yield relationship are ensured. Leaf water potential for irrigation timing purposes could be satisfactorily estimated by the model if calibrated over a sufficiently large range of leaf water potentials. The threshold potential for maximum water use efficiency was−1.82 MPa and for maximum yield−1.44 MPa.