Hypertension occurs at an earlier age, is more prevalent, and is more often complicated by target organ damage in African-Americans than whites. Reasons for this increased severity of hypertension in African-Americans remain obscure. Based on studies recently completed in their laboratory, the authors propose that greater sympathetic reactivity to stress and a greater prevalence of NaCl sensitivity contribute to the earlier development of hypertension in African-Americans. Using microneurography to record muscle sympathetic nervous system activity, it was found that normotensive blacks manifest greater increases in sympathetic activity to cold stress than normotensive whites. If true of other types of stressors, greater sympathetic reactivity would predispose blacks to the development of hypertension. Using a telemetry-based monitoring system, the authors recently reported that both spontaneously hypertensive rats and normotensive Wistar-Kyoto rats manifest acute sensitivity to high dietary NaCl ingestion, but that the Wistar-Kyoto rats are able to compensate, thereby avoiding sustained increases in blood pressure. Based on these animal studies, it is proposed that elevated nocturnal pressures observed in blacks by other investigators may reflect the greater prevalence of NaCl sensitivity in the black population. As in animal models of NaCl-sensitive hypertension, blacks may retain ingested NaCl, resulting in sustained increases in blood pressure.