Tissue-specific and time-dependent control of in vivo gene disruption may be achieved using conditional knockout strategies in transgenic mice. Fusion of mutant estrogen receptor ligand-binding domains to Cre recombinase (Cre-ER(T), MerCreMer) combined with cardiac-directed gene expression has been used to generate several cardiac-specific tamoxifen-inducible Cre-expressing mouse lines. Such mice have successfully been used to generate Cre-loxP-mediated gene disruption in an inducible manner in the myocardium in vivo. However, information is sparse regarding the tamoxifen dosage, the time course of gene disruption and whether different administration routes differ in efficiency in obtaining gene disruption in the myocardium. We have evaluated these parameters in Serca2 ( flox/flox ) Tg(alphaMHC-MerCreMer) transgenic mice (SERCA2 KO). Serca2 mRNA transcript abundance was used as a sensitive indicator of Cre-loxP-dependent gene disruption in the myocardium. We found that 2 i.p. injections of tamoxifen in oil (1 mg/day, approximate total dose 80 mg/kg) was sufficient for efficient gene disruption with maximal reduction of Serca2 mRNA as early as 4 days after tamoxifen induction. Moreover, a simple protocol using tamoxifen-supplemented non-pelleted dry feed p.o. was comparable to i.p. injections in inducing gene disruption. These improvements may significantly improve animal welfare and reduce the workload in the production of cardiac conditional knockout mice.