1. Synaptosomes have been isolated from guiuea-pig cerebral cortex and their appearance and enzymic and respiratory properties found to be similar to those reported for synaptosomes by other workers. 2. The effects of the convulsants, strychnine and pentamethylene tetrazol, and the anticonvulsants, phenobarbitone and acetazolamide, on some of the properties of synaptosomes have been examined. 3. The resting respiratory rate of synaptosomes was found to be insensitive to the convulsants and anticonvulaants and to a variety of other compounds which affect excitable neuronal membranes *in vivo*. The possible significance of these results has been discussed. 4. The incorporation of 14C in to synaptosomes from (U-14C)-glucose was inhibited by pentamethylene tetrazol and enhanced by phenobarbitone. 5. The uptake of xylose, which did not seem to occur by means of a simple diffusion process, was unaffected by phenobarbitone and pentamethylene tetrazol. 6. The concentration of xylose in the synaptosomes, once xylose uptake was complete, was unaffected by 2, 4-dinitroplienol. 7. The release of osmotically active constituents from synaptosomes suspended in warm sucrose was very much reduced by phenobsrbitone at relatively low concentrations and by acetaszolamide at concentrations which were substantially greater than those found 'in vivo'. 8. 45 per cent of the Na+ and 27 per cent of the K+ was lost to the medium if freshly prepared synaptosomes were incubated in sucrose at 25°C for 30 minutes, Phenobarbitone prevented the loss of these ions during the incubation. 9. Phenobarbitone had no effect on the release of xylose from synaptosomes which were suspended in warm sucrose.