The cardiovascular effects of nitroglycerine (TNG) were studied in 30 patients requiring controlled hypotension for intracranial aneurysm surgery. In patients "resistant" to TNG (n = 9), sodium nitroprusside (SNP) was used to supplement the TNG. With TNG alone at a total dose of 31 mg and a mean hypotension duration of 28 min, mean arterial pressure (MAP) decreased by 43%, from 78.3 to 44.4 mm Hg as a result of decreases in cardiac index (Cl) (18%) and systemic vascular resistances (SVR) (21%). Simultaneously, a moderate increase in (Cao2 - Cvo2) (21%), and a significant increase in plasma renin activity (90%) were observed. In patients "sensitive" to TNG, the MAP decreased by 54%, Cl by 27% and SVR by 35%; HR remained stable. In patients "resistant" to TNG, the decreases in Cl and SVR were less marked: 2% and 22% respectively; the observed increase in HR was 12%. When non-toxic doses of SNP were used (less than 2 micrograms kg-1 min-1), hypotension was caused by decreased SVR (31%) and increased Cl (8%). TNG alone can be used to produce controlled decreases in MAP to around 50 mm Hg, and in patients "resistant" to TNG, SNP can be added to increase the hypotensive effect.