We found in mice that repeated single daily subcutaneous (s.c.) isoproterenol (ISO) injections, like constant infusions using osmotic minipumps, caused increased biventricular mass or weight relative to body weight (VW/BW). We found that 5 (1/d) s.c. injections of 2, 10, or 20 microg/g body weight caused equivalent VW/BW increases as compared with 5-d infusions at 20 microg/(g.d)). While it is often presumed that ISO elicits hypertrophy by a direct effect on the myocytes, growth may also be secondary to systemic hemodynamic effects. The 2 modes of ISO administration had different effects on mean arterial blood pressure (MABP) and heart rate. Using telemetry we observed that single injections of ISO (0, 0.5, 2, and 10 microg/g) were associated with hypotension and tachycardia with a duration but not a magnitude that was dose dependent. MABP dropped rapidly to 60 mm Hg for more than 2 h at the highest dose. Constant s.c. infusion of ISO at 20 microg/(g.d) initially lowered MABP to about 70 mm Hg for 24 h. At 48 h MABP was normal, but rose 10 mm Hg higher than baseline by day 5. Thus, different routes of administration of ISO that cause comparable increases in VW/BW had different effects on MABP. Thus when evaluating mouse models of ISO-induced cardiac hypertrophy, both repeated daily injections or infusions can cause similar increases in VW/BW, but the daily doses that are required are not the same. Furthermore, these different routes of administration have different hemodynamic sequelae and could potentially evoke different cardiac phenotypes.