1. Calcium depletion from the medium of the isolated perfused kidney reduced renin release and renal perfusate flow.
2. Reintroduction of calcium increased renin release and perfusate flow.
3. The ionophore X 537 A (0·1–4 μmol/l) increased renin release both in the presence and absence of calcium in the medium.
4. The ionophore A 23187 (1–10 nmol/l) increased as well as decreased renin release. There was a positive correlation between direction of this effect and renal perfusate flow.
5. The results are compatible with the view that the effects of calcium and ionophores on renin release are the sum of direct and indirect effects of these agents, the predominant indirect effect being the modification of vascular tone.