p120 Ras GTPase‐activating protein (RasGAP) encoded by the rasa1 gene in mice is a prototypical member of the RasGAP family of proteins involved in negative‐regulation of the p21 Ras proto‐oncogene. RasGAP has been implicated in signal transduction through a number of cell surface receptors. In humans, inactivating mutations in the coding region of the RASA1 gene cause capillary malformation arteriovenous malformation. In mice, generalized disruption of the rasa1 gene results in early embryonic lethality associated with defective vasculogenesis and increased apoptosis of neuronal cells. The early lethality in this mouse model precludes its use to further study the importance of RasGAP as a regulator of cell function. Therefore, to circumvent this problem, we have generated a conditional rasa1 knockout mouse. In this mouse, an exon that encodes a part of the RasGAP protein essential for catalytic activity has been flanked by loxP recognition sites. With the use of different constitutive and inducible Cre transgenic mouse lines, we show that deletion of this exon from the rasa1 locus results in effective loss of expression of catalytically‐active RasGAP from a variety of adult tissues. The conditional rasa1 mouse will be useful for the analysis of the role of RasGAP in mature cell types. genesis 45:762–767, 2007. © 2007 Wiley‐Liss, Inc.