Clostridium botulinum type D toxin can, in certain preparations, cause a failure of the response to sympathetic nerve stimulation (Rand & Whaler, 1965). For the guinea-pig isolated hypogastric nerve-vas deferens preparation it might be argued that the block occurs in ganglion-cells within the hypogastric trunk, rather than at postganglionic nerve endings. Although the existence of ganglion-cells in the nerve trunk is accepted, their anatomical position in relation to the vas deferens itself is less clear. Ferry (1963Ferry ( , 1967 has shown that the hypogastric trunk has most of its ganglion-cells in the last 2 cm of the nerve trunk, mainly concentrated 0.4-0.9 cm from the muscle tissue. Thus, unless particular care is taken to have stimulating electrodes closer to the muscle than 0.4 cm, both preganglionic and postganglionic fibres will be stimulated.For the correct interpretation of results from experiments on the action of botulinum toxin on adrenergic nerve endings, the possibility that ganglion-cells are interposed between the point of nerve stimulation and the release of noradrenaline must be ruled out because transmission through such a ganglion cell to the postganglionic fibre is cholinergic and therefore toxin-sensitive. This is especially important when the results are contrary to the accepted belief that adrenergic nerves are not sensitive to botulinum toxin (Ambache 1949(Ambache , 1951Vincenzi, 1967).The experiments reported here were designed to exclude as completely as possible any preganglionic component: the results confirm that botulinum toxin blocks the response of the vas deferens preparation to postganglionic nerve stimulation. METHODS Hypogastric nerve-vas deferens preparationVasa deferentia, obtained from guinea-pigs weighing 300-650 g, were suspended in 50 ml. organ baths as described by Hukovi6 (1961); where possible, paired vasa deferentia from the same animal were used. The bath fluid (usually 40 ml.) was McEwen's (1956) solution, gassed with a mixture of 95% oxygen and 5% carbon dioxide and maintained at either 32' C or 36'-37' C. Isotonic contractions of the muscle were recorded on smoked paper by mean of a frontal writing lever.