Antidiuretic action of intravenous and intracerebral pyrogen in conscious rabbits.

1. In conscious rabbits an i.v. infusion of 30 or 60 microgram E. coli pyrogen/kg body wt. decreased urine flow from control of 1.13 +/- 0.11 (S.E. of mean) ml./min to 0.43 +/- 0.09 ml./min while urine osmolality increased from 212 +/- 16 to 679 +/- 82 m-osmole/kg H2O. Clearances of exogenous creatine and p-aminohippurate did not change significantly. 2. Plasma antidiuretic activity (rat bio-assay) increased from control of 15.5 +/- 4.2 to 56.8 +/- 5.5 muu./ml. at the top of antidiuresis. 3. Comparable urine flow and osmolality changes were evoked by a pyrogen dose of only 0.2 microgram per animal injected into the lateral cerebral ventricle. 4. The results suggest that in the rabbit pyrogen stimulates ADH release by an unknown central mechanism.