1. A two-chambered tentacular olfactometer was constructed (Fig. 1). This device permits the local delivery of odors to the tentacular sensory epithelium, with independent control over concentration on the left and right sides. The occurrence of discrete head turns during locomotion, subject to criteria of latency and extent, was indicative of olfactory orientation response. 2. Concentration-response profiles were obtained by delivering odors unilaterally (Fig. 8). The snails turned towards the vapors of amyl acetate, hexanoic acid, butyric acid and 2-octanol. The threshold sensitivities (typically, at 10−7 molar concentrations) were at least two orders of magnitude higher than comparable thresholds reported for other animals (Table 3). In contrast to the odors listed above, there were no reliable orientation responses for acetaldehyde, valeraldehyde, trans-2-hexenal or ethanol. 3. None of the odors reliably elicited turning away from the stimulus, despite the observation of tentacular retractions associated with some odors at high concentration, indicating aversive stimulation (Fig. 8, Table 2). 4. When amyl acetate was delivered at different concentrations to the two tentacles, the difference between the number of turns towards the strong side and towards the weak side increased as a function of the Weber fraction, with 0.75 being the minimum Weber fraction at which the difference was statistically significant (Fig. 9). 5. The methodology was relatively insensitive to inter-observer variations (Fig. 4). However, some factors which did affect the results in particular experiments were age of the snails (Fig. 5), hunger motivation (Figs. 6 and 7) and order of stimulus presentation (Fig. 6).