Current findings in male relaxin knockout mice indicate antifibrotic and positive lusitropic actions of relaxin on the myocardium. We investigated in 12-month-old male spontaneously hypertensive rats (SHR) and age-matched male Wistar-Kyoto rats (WKY) whether RLX shows antihypertrophic actions as well. SHR showed left heart hypertrophy and a selective elevation of left atrial and ventricular relaxin peptide which inversely correlated with the degree of hypertrophy. In adult rat cardiomyocytes, relaxin blunted phenylephrine-induced hypertrophy by inhibiting activated ERK-1/2 kinases. In conclusion, endogenous myocardial relaxin, upregulated in left heart hypertrophy, exerts antihypertrophic effects by inhibition of activated ERK-1/2 kinases.