A recent study published in Science showing that urinary OT levels are elevated in association with mutual gaze between dogs and their owners suggests these two lines of research may be converged on the target of PTSD. The investigators will compare the pattern of results of tests of attention bias and startle induced by intranasal OT (vs placebo) to those the investigators have shown to be induced by the presence (vs absence) of a service dog in Veterans diagnosed with PTSD.