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1. A study has been made of the mechanism by which injected disodium GTP stimulates the ouabain-insensitive Na efflux in single muscle fibres from the barnacle, Balanus nubilus. 2. Injection of GTPNa2 causes a stimulatory response which is usually transitory and almost completely reversed by injecting MgCl2 (but not KCl). 3. Injected 5'-guanylylimidodiphosphate, Gpp(NH)p, mimics this action of GTP but the reversal seen with injected Mg2+ is less pronounced. 4. (i) Pre-treatment of these fibres with verapamil reduces the size of the stimulatory response to GTP and Gpp(NH)p. (ii) Pre-injection of protein kinase inhibitor (PKI) or regulatory subunits reduces the response as well. (iii) Pre-treatment with imipramine or trifluoperazine reduces the response to injected GTP; in combination with verapamil, a greater reduction in response is seen. 5. Injection of EDTA leads to a stimulatory response which is transitory. This response is largely abolished by verapamil. 6. Injection of cholera toxin leads to a sustained stimulatory rather than a transitory response. GTP or Gpp(NH)p when injected following peak stimulation by cholera toxin leads to a moderate sustained stimulation. 7. These results support the view that the stimulatory response to injected GTPNa2 is the result of activation of Ca2+ channels and of increased availability of GTPMg and that these two conditions bring about activation of adenylate cyclase and hence activation of cyclic AMP-protein kinase by newly formed cyclic AMP.