The influence of temperature on membrane–prazosin interactions was studied. Prazosin, a quinazoline derivative of 2-furoylpiperazine, is a classic antihypertensive and antiarrhythmic drug. A mixed cholesterol/phospholipid monolayer at the water/air interface was employed as a simplified biomembrane model. Brewster angle microscopy (BAM) was used to visualize the monolayer morphology. It was found that prazosin penetrates Langmuir monolayers and modifies the interactions between membrane components, causing monolayer fluidization. An increase in temperature facilitates penetration of prazosin into the monolayers. Prazosin interacts preferentially with phosphatidylcholine and modifies the morphology of the condensed phase domains of DPPC. In the presence of prazosin, monolayers collapse at lower surface pressures. The difference between the collapse pressures of monolayers on water with and without prazosin increases with temperature.