Current models of essential hypertension have limited comparisons to humans due to differences in posture and periods of activity. The annual cycle of spontaneous hypertension in male Chlorocebus aethiops sabaeus was assessed in an outbred research colony. Using plethysmography, systolic (SBP) and diastolic blood pressures (DBP) of adult male African Green, or vervet, monkeys (n=163) were measured in May/June after ketamine sedation (~10‐15mg/kg). Blood pressure phenotypes were characterized as hypertensive (HT: SBP 蠅 140mmHg) or normotensive (NT: SBP 蠄 120mmHg). SBP and DBP identified 33% (n=53) HT (average SBP 169.04±24.19mmHg) and 41% NT (average SBP 101.32 ±13.28mmHg, n=66) animals. Borderline hypertension (BHT) was identified with an average SBP of 129.09±5.85mmHg (n=44). A randomized group of animals (n=51) were re‐measured in September/October. HT monkeys (n=21) decreased blood pressure from 174.14±27.48mmHg to 128.61±28.48mmHg (p<0.05). SBPs of NT (n=21) and BHT animals (n=9) were unchanged (p>0.05). A third of adult male vervet monkeys in this colony exhibit spontaneous hypertension with unique seasonal variation compared to NT or BHT animals. This model of hypertension in an Old World non‐human primate is highly translational to human essential hypertension and allows for new research directions in a novel genetically similar species. Grant Funding Source : Supported by Behavioural Science Foundation