The influence of drought stress on barley (Hordeum vulgare) has been investigated. The experiments were conducted with seedlings of barley—by the application of combined drought and temperature stresses. Here we showed that combination of drought and high temperature inflicted more severe damage to plants than the drought and low temperature stress. The temperature stress triggered more drastic changes in plant morphology/physiology and biochemistry than the drought stress. Interestingly, plants exposed to high temperature exhibited significant reduction of shoot and root aldehyde oxidase (AO) activity. Moreover, increased temperature resulted in lower levels of reactive oxygen species (ROS), while drought stress had an opposite effect on ROS accumulation and AO activity. This is the first demonstration of inhibition of plant AO and ROS in response to heat stress. The combined heat and drought stresses resulted in increased activity of catalase (CAT) and superoxide dismutase (SOD) in roots but not in shoots. Our findings indicate that heat and drought stresses may induce activation of different antioxidant enzymatic defense mechanisms and heat stress significantly affects enzymes responsible for the ROS accumulation in plants.