Bladder cancer has been associated with chronic arsenic exposure. Monomethylarsonous acid [MMA(III)] is a metabolite of inorganic arsenic biotransformation and has been shown to transform an immortalized urothelial cell line (UROtsa) at a concentration 20-fold less than arsenite. MMA(III) was used as a model arsenical to examine the mechanisms of arsenical-induced transformation of the urothelium. A microarray analysis was performed to assess the transcriptional changes in UROtsa during the critical window of chronic MMA(III) exposure that leads to transformation at three months time. The analysis revealed only minor changes in gene expression at one and two months of exposure, contrasting with substantial changes observed at three months of exposure. The gene expression changes at three months were analyzed showing distinct alterations in biological processes and pathways such as a response to oxidative stress, enhanced cell proliferation, anti-apoptosis, MAPK signaling, as well as inflammation. To address the lack of information between two and three months of exposure -- the critical period of transformation -- the expression of selected pathway marker genes were measured by PCR array analysis on a weekly and monthly basis. A very similar pattern of altered expression of these genes was observed when compared to microarray results, and suggested early perturbations in cell signaling cascades, immunological pathways, cytokine expression, and MAPK pathway, are particularly important in driving malignant transformation. These results showed a strong association between the acquired phenotypic changes that occurred as early as one to two months of chronic MMA(III) exposure, and gene expression patterns that are indicative of the earliest stages in carcinogenesis. Additionally, studies on the effects of withdrawal of arsenical were also conducted and showed that phenotypic changes persisted even in the absence of arsenical; that gene expression patterns of pathway marker genes, those that showed significant alterations between 3 and 6 months of exposure, appeared to normalize after withdrawal of the arsenical.