Chloride fluxes in crab muscle fibres

1. Isotopic techniques were used to study the efflux of Cl − from single cannulated muscle fibres of the crab Maia squinado . 2. The efflux of 36 Cl − was found to be exponential with steady rate constants ranging from 0·01 to 0·07 min −1 . The steady efflux was ca . 1000–2000 p M cm −2 sec −1 . The efflux was slightly depressed by K + ‐free saline and was unaffected by 40 m M ‐K + saline and NO 3 − saline. The influx and efflux of Cl − were presumed independent. 3. The membrane conductance calculated from 36 Cl − flux data was consistent with that determined by the method of electrotonic spread for other marine crustacean muscle fibres. Cl − accounted for most of the membrane conductance.