Tissue distribution and quantitative analysis of small molecules is a key to assess the mechanism of drug action and evaluate treatment efficacy. The prodrug irinotecan (CPT-11) is widely used for chemotherapeutic treatment of colorectal cancer. CPT-11 requires conversion into its active metabolite SN-38 to exert the desired pharmacological effect. MALDI-Fourier transform ion cyclotron resonance (FT-ICR) and MALDI-time-of-flight (TOF) mass spectrometry imaging (MSI) were performed for detection of CPT-11 and SN-38 in tissue sections from mice post CPT-11 injection. In-depth information was gained about the distribution and quantity of drug compounds in normal and tumor tissue. The prodrug was metabolized, as proven by the detection of SN-38 in liver, kidney and digestive tract. In tumors from genetic mouse models for colorectal cancer ( Apc 1638N/wt x pvillin- Kras V12G ), CPT-11 was detected but not the active metabolite. In order to correlate drug distribution relative to vascularization, MALDI data were superimposed with CD31 (PECAM-1) immunohistochemistry. This analysis indicated that intratumoral access of CPT-11 mainly occurred by extravasation from microvessels. The present study exploits the power of MALDI MSI in drug analysis, and presents a novel approach to monitor drug distribution in relation to vessel functionality in preclinical and clinical research.