A specific chloride channel blocker, anthracene 9-carboxylic acid was locally applied for 8-9 days on the extensor digitorum longus muscle of 7-8-day-old rats. The effects of chronic anthracene 9-carboxylic acid treatment on muscle development, were evaluated in vitro on the electrical properties with intracellular microelectrodes and in vivo on the contractile parameters by recording isometric concentrations. Our data show that the treatment prevented the normal development of chloride conductance so that by 15 days of age it was 45% lower in fibers of the treated muscles when compared to age-related control fibers. Potassium conductance was not significantly changed by the treatment. In vivo the anthracene-9-carboxylic acid-treated muscles were slower to contract and relax; having a 20% slower time to peak twitch force and time of half relaxation. These muscles were also 32% less fatiguable with respect to the controls. Moreover, in most of the treated muscles tetanic contractions during high-frequency stimulation were not maintained. The block of chloride channels in developing striated fibers appears to affect the differentiation of specific properties of fast skeletal muscle such as the speed of contraction.