An infusion of 0.1% trimetaphan was administered to eight cats with artificially increased intracranial pressure (ICP) in order to decrease their mean arterial pressure (MAP) from 121 +/- 9.5 (SEM) to 58 +/- 4.6 mm Hg in less than 1 min. All cats developed an increase in intracranial pressure (ICP) (from 16 +/- 1.4 to 23 +/- 3.2 mm Hg) accompanied by a partial rebound in MAP. Eight additional cats received 0.1% trimetaphan to decrease their MAP from 128 +/- 13.4 to 52 +/- 8.1 mm Hg over more than 2 min. Four of these cats followed the same pattern, with ICP increases from 19 +/- 1.1 to 31 +/- 3.9 mm Hg, while in the other four ICP did not change. In nine of the 12 cats with an ICP increase, that increase was initiated before the partial MAP rebound. We conclude that trimetaphan causes clinically significant ICP increases in cats with increased ICP, that partial rebound in MAP frequently exacerbates these increases in ICP, and that rapid induction of hypotension tends to increase the frequency with which trimetaphan increases ICP.