Phosphorylated ERK expression has been demonstrated in the central and peripheral nervous system after various stimuli including visceral stimulation. Changes in the activation (i.e., phosphorylation) of extracellular signal-regulated kinases (pERK) were examined in the urinary bladder after (4 hour (hr; acute), 48 hr (intermediate) or chronic (10 day) cyclophosphamide (CYP) treatment. CYP-induced cystitis significantly (p 0.01) increased pERK expression in the urinary bladder with intermediate (48 hr) and chronic CYP-treatment. Immunohistochemistry for pERK-immunoreactivity revealed little pERK-IR in control or acute (4 hr) CYP-treated rat urinary bladders. However, pERK expression was significantly (p 0.01) upregulated in the urothelium after 48 hr or chronic CYP-treatment. Whole mount preparations of urothelium/lamina propria or detrusor smooth muscle from control (non-inflamed) rats showed no pERK-IR in PGP9.5 labeled nerve fibers in the suburothelial plexus. However, with CYP-treatment (48 hr, chronic), a few pERK-IR nerve fibers in the suburothelial plexus of whole mount preparations of bladder and at the serosal edge of urinary bladder sections were observed. pERK-IR cells expressing the CD86 antigen were also observed in urinary bladder from CYP-treated rats (48 hr, chronic). Treatment with the upstream inhibitor of ERK phosphorylation, U0126, significantly (p 0.01) increased bladder capacity in CYP-treated rats (48 hr). These studies suggest that therapies targeted at pERK pathways may improve urinary bladder function in CYP-treated rats.