Abstract Body: Autonomic failure (AF) is characterized by orthostatic hypotension (OH), but most of these patients also have nocturnal supine hypertension (SH), which has been associated with end-organ damage, pressure natriuresis, and worsening of daytime OH. About 30% of patients with SH, however, have a preserved nocturnal BP dipping that is likely associated with a morning BP surge (MBPS). To determine the pattern of MBPS and its association with dipping, natriuresis, and morning OH, we studied 92 AF patients (67±1 yr, 59% males; 32 multiple system atrophy and 60 peripheral autonomic failure [pAF]). Patients were studied off BP medications and placed on a low monoamine, caffeine-free diet containing 150 mEq/d sodium and 70 mEq/d potassium. Nocturnal BP was assessed every 2 hours from 8pm-8am, followed by a 10-minute standing test. Urine was collected for 24 hours and divided into two 12-hour periods. As expected, we found that dipping (systolic BP [SBP] fall ≥10% during nighttime sleep [12am-6am] from baseline [8pm to 10pm]) occurred in 33% of patients, with an average decrease of 35±5 mmHg at 4am. The MBPS (SBP increase at 8am from the average of 3 nighttime SBPs centered on the lowest one) was 10±2 mmHg (range -83 to 77 mmHg; MBPS≥ 20 mmHg, 23%). MBPS was negatively correlated with the magnitude of dipping (r=-0.328, p<0.001; dippers 18±3 vs. nondippers 6±3 mmHg; p=0.010) and mean nighttime SBP (r=-0.230, p=0.028). The night-to-day ratio of sodium (r=-0.417, p=0.002) and urine (r=-0.292, p=0.037) excretion was also negatively correlated with MBPS, indicating that patients with greater MBPS had decreased natriuresis during the night compared to the day. In the morning, a greater MBPS was associated with a larger SBP drop on standing (r=-0.242, p=0.034). No associations were found between MBPS and age, sex, history of essential hypertension, or the underlying diagnosis of AF. To assess the hemodynamic mechanisms of the MBPS, we studied 17 AF patients (67±2 yr, 14 males, all pAF). We found that MBPS (13±5 mmHg, range -18 to 56 mmHg) was associated with an increase in systemic vascular resistance (SVR, r=0.777, p=0.01) and a higher augmentation index (Aix@75, r=0.516, p=0.049). In conclusion, a subset of AF patients had a preserved dipping pattern and an MBPS despite severe impairment of BP regulation. MBPS was associated with increased SVR, lower nighttime SBP, and lower night-to-day natriuresis, but the higher supine SBP in the morning was associated with worse OH.