An understanding of the regulation of calcium-activated chloride channels (CaCC) is important in the development of pharmacotherapies for cystic fibrosis. Protein kinase C and Ca²⁺/Calmodulin protein kinase II have been implicated in the regulation of CaCC in several cell lines (1, 2); however, little is known about the involvement of protein phosphatases in CaCC regulation. The iodide efflux assay was used to investigate the effect of protein phosphatase inhibitors on the UTP-evoked response in CFPAC-1 monolayers. Initially, okadaic acid (OA), which can inhibit protein phosphatases (PP) 1, 2A and 2B at specific concentrations, was assessed. OA (5nM) reduced the UTP-response by 48%, from 12.6 ± 1.7% to 6.5 ± 0.8% (n=6, p<0.001, 2-way repeated measures ANOVA). OA (50nM) further reduced the UTP-induced efflux by 64%, from 13.7 ± 2.4% to 5.0 ± 0.3% (n=6, p<0.001). However, 100nM OA only decreased the UTP-mediated response by 25% from 10.9 ± 1.2% to 8.2 ± 1.3% (n=6, p0.05). This would suggest that PP2A is not involved in the regulation of CaCC, as this concentration is around the IC₅₀ value for PP2A. Endothall (100μM) reduced the UTP-induced efflux by 29%, from 10.0 ± 1.4% to 7.1 ± 1.5% (n=8, p<0.001). Taken together this data indicates that PP1 could be involved in the regulation of CaCC. The other protein phosphatase that could be implicated from the initial OA results is PP2B, also known as calcineurin (CaN). The major CaN inhibitors are the immunosuppressant drugs cyclosporin A (CsA) and FK506, which bind specifically to CaN (3). CsA (10nM) reduced the UTP-induced efflux by 64%, from 8.4 ± 0.8 to 3.0 ± 0.4 (n=4, p<0.001). FK506 (10μM) reduced efflux by 43%, from 7.0 ± 1.0% to 4.1 ± 1.1% (n=8, p0.05) suggesting no involvement of mTOR and supporting CaN-dependent FK506 activity. Taken together these data suggest the involvement of PP1 and PP2B/CaN in the regulation of CaCC in CFPAC-1 cells.