An electrophysiological study of odour similarities of homologous substances.

1. The activity of single bulbar units in the olfactory system of the frog was recorded extracellularly by means of micro-electrodes. The electro-olfactogram was recorded simultaneously from the receptor epithelium.2. The olfactory epithelium was stimulated with substances of homologous series of normal aliphatic alcohols, acetates and ketones.3. The effect on a bulbar unit was classified as excitatory or inhibitory, and the chi-square values calculated with one degree of freedom for all pairs in a given series. The statistical values obtained indicated the degree of similarity in olfactory stimulative properties between the odours.4. The results show that the degree of similarity in olfactory properties is greatest among neighbouring substances and gradually decreases with increasing separation in chain length.5. Significant rank correlation coefficients were found between the chi-square values and the molecular weight ratios for alcohols and acetates. The results are discussed in relation to psychophysical findings obtained with the same substances.