© 2015, Springer Science+Business Media Dordrecht. 24-epibrassinolide (EBL) is a growth regulator that promotes crop growth and yield, especially under water stress, but its effect on grass pea (Lathyrus sativus L.), a legume crop widely recognized as adapted to rainfed environments, is unknown. The aim of the present study was to investigate the effect of exogenously-applied EBL on growth, yield and accumulation of ß-N-oxalyl-L-a, ß-diaminopropionic acid (ß-ODAP)—a neurotoxin that can induce lathyrism in animals and humans—in grass pea under well-watered (WW) and water-stressed conditions. The first experiment conducted in a growth chamber showed that EBL application increased plant height and leaf area of well-watered [WW, 85 % field capacity (FC)] grass pea seedlings and in those in which the soil dried from 85 to 30 % FC. In two pot experiments conducted under a rainout shelter, three water regimes—soil water contents maintained at (1) 85 or 80 % FC, (2) 50 % FC, and (3) 35 % FC—were imposed. In the first rainout shelter experiment, root/soil drenching with EBL significantly increased aboveground dry weight (DW) and water use efficiency (WUE) compared with plants without EBL at 85 and 50 % FC, but did not significantly affect grain yield. In a second rainout shelter experiment, water treatments were imposed from the vegetative phase to maturity and EBL treatment significantly increased aboveground DW, grain yield, ß-ODAP concentration and amount, and WUE in the 80 and 50 % FC water regimes. It is concluded that exogenously-applied EBL can stimulate growth and WUE in grass pea when WW and moderately water stressed, and also stimulates ß-ODAP production such that the concentration in the grain is similar or even higher.