Intracramal pressure (i.c.p.) was measured in 11 normocapnic dogs anaesthetized with thiopentonc and morphine and ventilated with oxygen-enriched air. The changes in i.c.p. in response to infusions of enher sodium mtroprusside (SNP) 10 μ kg-1 min-1 (five anjmaln) or glyceryl trinitrate (TNG) 30 μg kg-1 min-1 (six animals) i.v. were recorded at a normal baseline i.c.p. and, in the same animalu, once the baseline i.c p. had been increased to 20mmHg and 40mmHg. Modest decreases (10-20%) in mean arterial pressure (m.a.p.) were noted during the infusions and the concomitant increases in i.c.p. in every animal were significant at each value of i.c.p. (P<0.05). Acute pupillary dilatation was noted during SNP infusion, and further observations of pupillary changes occurring during infusion of both SNP and TNG at greater i.c.p. (60 mm Hg) emphasize the existence of trans-tentorial pressure gradients and their aggravation by the use of these agents in the presence of a very high i.c.p.