Glycerolformal (CAS 5464-28-8; a mixture of 1,3-dioxan-5-ol and 1,3-dioxolane-4-methanol) used as an organic solvent or vehicle for drugs has been shown to possess its own toxicopharmacological activities. The aim of the present investigation was to determine the effects of glycerolformal on sympathetic neurotransmission in the isolated rabbit heart. At concentrations between 0.05 and 1 mmol/l glycerolformal inhibits both the neuronal and extraneuronal uptake of noradrenaline and its metabolism degradation which could explain the initial positive inotropic action of glycerolformal on the heart by increasing noradrenaline concentration in the synaptic cleft. However, the preponderant effect of glycerolformal was an inhibition of noradrenaline release, resulting in a myocardial depression which may explain the hypotension observed in the anesthetized rat. Hence, it is important to take into account the interference effects of glycerolformal with other molecules, when used as solvent or vehicle for drugs.