In 15 cats under chloralose-urethan anaesthesia carotid sinus area was arterially isolated and perfused with artificially pulsatile pressures. Solutions of NaCl, of various osmolality, mannitol, glucose and bicarbonate were locally perfused through the isolated carotid area and the single baroreceptor and chemoreceptor fibre activity was recorded with standard technique. An increase in the osmolality of the perfused fluid evoked an increase of the single baroreceptor discharge. Each pulse pressure produced more spikes of higher frequency in big A type baroreceptor fibres. Clear effects were observed during perfusion of 300 mM NCl solutions. Solutions of higher osmolility transformed rhythmical pulse pressure discharge into continuous baroreceptor firing. The excitatory effects of hyperosmolar solutions were reversible after switching to the perfusion fluids of normal osmolality. An increased activity was also observed in the single carotid chemoreceptor fibres. It is concluded that both baro- and chemoreceptors are sensitive to osmotic stimulation.