External intercostal muscle biopsies from normal and congenitally rnyotonic goats were studied in vitro at 30~ using a two-microelectrode square-pulse cable analysis assisted by computer. The resting chloride conductance (Go1) was estimated from the difference between the mean membrane conductance in chloride-containing and chloride-free bathing media. The protein kinase C (PKC) activator, 4-fi-phorbol-12,13-dibutyrate, (0.1-2.0 gM) blocks a maximum of 76% of Gd in normal goat fibers and induces myotonic hyperexcitability similar to that of congenitally myotonic goat fibers. The Gotblock was partially antagonized by pretreatment with the PKC inhibitor, staurosporine (10 ~tM). The "inactive" 4-ephorbol-12,13,didecanoate had no effect at 50gM, whereas the "active" 4-fi isomer blocked 41% Go1 at I ~tM. The nearly absent Gox of congenitally myotonic goat fibers was not restored by treatment with high concentrations of the PKC inhibitors staurosporine, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), or tetrahydropapaveralone (THP). Also, forskolin and cholera toxin, which may increase cyclic adenosine monophosphate (cAMP) levels, or the R(+) clofibrie acid enantiomers and taurine, which increase Go1 in normal fibers, were also unable to restore Gel in myotonic goat fibers. The data suggest that PKC may be a chloride channel regulator in normal goat skeletal muscle fibers, however the molecular defect of congenitally myotonic fibers does not appear to be due to excessive activity of PKC.