The effects of iv succinylcholine (SCh) on the electroencephalogram (EEG), cerebral blood flow (CBF), cerebral metabolic rate (CMRo2), intracranial pressure (ICP), central venous pressure (CVP), and mean arterial pressure (MAP) were tested in halothane-anesthetized dogs. Six dogs were maintained at 0.87 ‡ 0.00% (mean ‡ SE) expired halothane (1.0 MAC) and received both SCh 1.0 mg. kg−1and lactated Ringer's solution placebo 0.05 ml. kg−1. Fasciculations began 24 ‡ 4 s after iv SCh. Fasciculations were followed by immediate EEG arousal in five of six dogs and increases in CBF in all six. Average CBF was 151 ‡ 14% of control for the 0–15 min measurement period and 127 ‡ 7% of control for the 15–30 min period. Both were significantly greater than pre-SCh control values and placebo group values. Peak CBF of 177 ‡ 19% of control occurred 3 min after iv SCh and was accompanied by a peak ICP of 435 ‡ 131% of control. ICP values were significantly different between SCh and placebo treatments only during the periods of greatest CBF (1 to 5 min after iv SCh). Average Paco2values after iv SCh were significantly greater than pre-SCh control values and placebo values during each 15-min measurement interval. Average Paco2was 116 ± 2% of control during the 0–15 min measurement period, 114 ± 2% of control during the 15–30 min period, and 109 ± 1% of control during the 30–45 min period. CVP, MAP, and CMRo2did not significantly change after iv SCh. In two dogs maintained at 1.32 ± 0.01% expired halothane (1.5 MAC), SCh 1.0 mg. kg−1produced Paco2changes comparable with those in dogs maintained at 1.0 MAC halothane without comparable changes in CBF, ICP, or EEG. In an additional two dogs receiving pancuronium 0.2 mg.kg−1and 1.0 MAC halothane, SCh had no meaningful effect on any variable measured. The authors conclude that iv SCh increased ICP in the dog secondary to increases in CBF. They hypothesize that the CBF increases are related primarily to SCh-induced increases in afferent muscle spindle activity and secondarily to increases in Paco2.