Hydroxocobalamin (HOCb), when mixed with sodium nitroprusside (SNP) in a 10:1 or 1:1 molar ratio and injected (i.v.) into the anaesthetized rat, prolonged the depressor response to SNP by 25-50%, but did not affect the degree of blood pressure lowering. Both the 'onset' and 'offset' components of the response were prolonged. Injecting [14C]SNP along with a 10-fold molar excess of HOCb resulted in a 2- to 3-fold elevation of plasma radioactivity which was maintained during the first 10 min of a 40 min experimental period. These effects of HOCb on the pharmacodynamics and pharmacokinetics of SNP are probably due to complex formation between the two compounds. Sodium thiosulphate (ST) added to SNP (12:1 molar ratio) had no effect on the depressor response to SNP. This mixing of ST and SNP had a less-marked influence on the plasma [14C] SNP-derived radioactivity than occurred with HOCb. There was no initial elevation of radioactivity, but the levels were raised by 50-60% at 4, 6 and 10 min. Since the depressor response to SNP was unaffected by ST, it is presumed that the higher concentrations of radioactivity were due to inactive degradation products rather than the active species itself.