Newborn spontaneously hypertensive rats (SHR), compared with Wistar-Kyoto (WKY) controls, usually show cardiac and renal hyperplasia. To determine whether these anomalies are common to genetically hypertensive rats, we examined newborn rats from models of essential hypertension (Kyoto, Montreal, Dunedin and Lyon strains of hypertensive rats), renal hypertension (Milan strain of hypertensive rats) and experimental hypertension [deoxycorticosterone acetate (DOCA)-salt hypertensive Wistar rats]. The hearts, kidneys and livers of these newborns were collected on site at various centres and sent to Montreal for protein and DNA determinations. The results showed that protein and DNA, corrected for body weight in models of essential hypertension, were increased in the heart and kidney, and normal or decreased in the liver at birth. This pattern differed in offspring of the Milan strain (renal hypertension) and of DOCA-salt hypertensive animals (experimental hypertension). Since cardiac and renal hyperplasia associated with the hypertensive trait in four different genetic models of spontaneous hypertension was distinct from that observed in renal and experimental hypertension, it is conceivable that a specific cardiovascular growth pattern is a reflection not of a simple linkage or consequence, but of a causal association with spontaneous hypertension.