The purpose of this study was to examine the effect of ammonium chloride ingestion on the hypercalcemic effect of parathyroid hormone in vivo. Thyroparathyroidectomized rats were given 1.5% ammonium chloride for 5–6 days. Ingestion of ammonium chloride increased serum calcium, but also significantly enhanced the calcium elevating effect of injected parathyroid extract. This result is compatible with a proposed hypothesis that the calcium mobilizing function of the parathyroid hormone may be enhanced by the hormone’s own influence on systemic hydrogen ion concentrat