Reduction of carbon tetrachloride toxicity by prior administration of a single small dose in mice and rats.

Mice were given progressively smaller doses of carbon tetrachloride (CCl4) and at intervals later the LD50 of a second dose was determined. The LD50 was greater than in previously untreated mice as soon as 10 min after the first dose, increased to a maximum between 12 and 24 h that was maintained for about 3 or 4 days, after which the LD50 returned to normal by the 7th day. The maximum LD50 reached was dependent on the first dose, but even a minute first dose, 0-004 ml/kg, had a significant effect. The same phenomenon was confirmed in rats. Administration of promethazine before doseing with CCl4 did not have this effect, nor did an ether anaesthetic.