This study investigated the physiological response of four cowpea varieties to drought and also the contribution of solutes to changes in leaf osmotic potential at full hydration (Fp100). The four cowpea varieties of varying drought tolerance (TVU 7778 - highly drought susceptible, IT-97K-568-19 -highly drought resistant, Bambey 21 - fairly drought susceptible and Mouride - fairly drought resistant) were grown in the field using a split plot in a randomized complete block design replicated thrice. The cowpea varieties were subjected to two levels of watering regimes (well-watered and water stressed) imposed on the plant at third week of age. Plant height, shoot dry matter production, stomatal diffusive resistance, transpiration rate, leaf relative water content (RWC), Fp100, osmotic adjustment (OA) and solute accumulation (K+, Cl-, total soluble carbohydrate and starch) were measured and the data subjected to analysis of variance at probability level of 0.05. The results showed that water stress significantly reduced plant height, shoot dry matter production and transpiration rate of all the varieties as the stress period increased but significantly increased stomatal diffusive resistance. It also significantly decreased leaf RWC of IT-97-568-19, Bambey 21 and Mouride by 27%, 16% and 29%, respectively but had no significant effect on that of TVU 7778. The Fp100 of IT-97K-568-19 was significantly decreased by water stress while that of the other varieties were not significantly affected. By the end of the stress period, IT-97K-568-19 had significantly the highest level of OA (0.97MPa), this was closely followed by Mouride (0.20 MPa) and the least was TVU 7778 (0.05 MPa). With the exception of Cl- that was significantly increased in IT-97K-568-19 due to water stress, the other solutes did not seem to contribute to decrease in osmotic potential found in this variety. IT-97K-568-19 therefore adjusted osmotically more than the other varieties and Cl- was the major solute that accounted for its high OA.