The effects of long-term diltiazem treatment on hemodynamic and cardiovascular characteristics were investigated in young spontaneously hypertensive rats (SHR), Wistar-Kyoto rats (WKY) and their respective untreated controls. The drug was administered to treated rats over a period of 24 weeks. Body weight, left ventricular weight, mean arterial pressure (MAP), heart rate, max dp/dt or maximum velocity of the contractile element (Vmax) were not significantly different in diltiazem-treated SHR and untreated SHR. In diltiazem-treated SHR, cardiac index (CI) and stroke volume index (SI) were significantly increased and total peripheral resistance and the index of left ventricular compliance (delta P/delta V) were significantly decreased compared with untreated SHR. Left ventricular pumping ability in treated SHR was higher than that in untreated SHR, despite the low dose of diltiazem given. However, there was no significant difference between treated and untreated WKY. Long-term diltiazem treatment did not affect left ventricular function or biochemical properties in SHR and WKY. These data suggest that long-term diltiazem treatment improves pump function in SHR without changing blood pressure.