Summary: Ligand‐dependent site‐specific recombinases are powerful tools to engineer the mouse genome in specific somatic cell types at selected times during pre‐ and postnatal development. Current efforts are primarily directed towards increasing the efficiency of this recombination system in mice. We have generated transgenic mouse lines expressing a tamoxifen‐activated Cre recombinase, CreER^T2, under the control of the smooth muscle‐specific SM22 promoter. Both a randomly integrated transgene [SM‐CreER^T2(tg)] and a transgene that has been “knocked in” into the endogenous SM22 locus [SM‐CreER^T2(ki)] were expressed in smooth muscle‐containing tissues. The level of CreER^T2 expression and tamoxifen‐induced recombination was lower in SM‐CreER^T2(tg) mice compared with SM‐CreER^T2(ki) mice. Whereas no recombinase activity could be detected in vehicle‐treated SM‐CreER^T2(ki) mice, administration of tamoxifen induced the excision of a loxP‐flanked reporter transgene in up to 100% of smooth muscle cells. The recombined genome persisted for at least four months after tamoxifen treatment. SM‐CreER^T2(ki) transgenic mice should be useful to study the effects of various somatic mutations in smooth muscle. genesis 28:15–22, 2000. © 2000 Wiley‐Liss, Inc.