Blood cyanide (HCN) or plasma thiocyanate (SCN) concentrations, or both, were measured in 30 patients (ages 11 months-72 yr) receiving sodium nitroprusside (SNP) for 12-314 h. Sequential measurements in three of these patients (infused 5, 12 and 13 days) showed that HCN concentrations varied with dose rate, while SCN concentrations increased linearly with increasing SNP dose. The accumulated data confirmed that the rate of administration (0.3-6.5 micrograms kg-1 min-1) determined the plasma HCN concentrations (0-3.8 mumol litre-1; y = 0.267 X -0.0733; r = 0.64; n = 51; P less than 0.001). Thus, if prolonged exposure to plasma HCN concentrations greater than 1 mumol litre-1 is to be avoided, the maximum safe sustained dose rate of SNP will lie near to 4 micrograms kg-1 min-1. Likewise, the SCN results (30--880 mumol litre-1) confirmed the close relationship between plasma concentrations and the total SNP dose (0.44-32.9 mg kg-1; y = 24.6x + 74.9; r = 0.95, n = 51, P less than 0.001). Therefore, we suggest that, to avoid SCN toxicity (plasma SCN greater than 1.75 mumol litre-1), in the absence of SCN monitoring, the total SNP dose should be less than 70 mg kg-1 in patients with normal renal function.