To characterize the behavioral consequences of OMP gene deletion on odor processing we assessed the ability of OMP-null animals to acquire an air vs odor discrimination for five odorants, and determined whether OMP-null animals differed from controls in their threshold sensitivity to the odorant propanol. On average, control and OMP-null animals did not differ in the number of testing sessions needed to achieve criterion performance on each discrimination problem (2.04 vs 1.68, respectively; t=0.83, p=0.41). However, null animals were significantly less sensitive to the odorant propanol (3.01 x 10(-8) vs 1.06 x 10(-5), respectively; t=4.09, p=0.015). These in vivo behavioral results provide support for the hypothesis that OMP plays a modulatory role in the odor detection/signal transduction process.