Growth of renal cysts results from both renal epithelial cell proliferation and fluid accumulation within the cyst lumen, and cAMP has a central role in this process. Inhibition of G protein coupled P2Y receptors upstream of cAMP can reduce growth of MDCK-derived cysts (1). Activation of P2Y receptors can increase intracellular [Ca²⁺] and stimulate cAMP activity, both of which are important signalling events for the extracellular signal-related kinase (ERK) cascade. We have investigated the activity of ERK on growth of MDCK-derived cysts. MDCK cells were cultured in collagen gel in the presence of the cAMP agonist forskolin to stimulate cyst formation. The ERK pathway was inhibited upstream using the MEK inhibitors PD98059 (50 μM) or U0126 (10 μM). Cyst diameter was measured directly from sequential photographs and cyst volume calculated. MDCK cysts were harvested from collagen gels by centrifugation (12 000 g, 5 min), washed 3 times in Dulbecco’s PBS and the pellet resuspended in Ripa buffer. Fifty µg of protein was electrophoresed on 12% SDS-PAGE gels and activity of ERK was measured by immunoblot. Results are expressed as means ± SEM of n observations for a total of three experiments per treatment. One-way ANOVA was used to compare data sets and differences were considered statistically significant if p<0.05. In control MDCK cysts, growth rate was 1.2 ± 0.16 nl/day (n=28 cysts). Inhibition of the ERK pathway resulted in a 90% reduction in cyst growth (PD98059 0.12 ± 0.02 nl/day, P < 0.001, n=36 and U0126 0.14± 0.02 nl/day, P < 0.001, n=41). Phospho-ERK activity increased over time on days 6 and 9, and was maximal on day 12 when compared with non-phosphorylated ERK. These data strongly suggest the ERK signal transduction pathway for cell proliferation is a key component of growth of MDCK cysts.