Negative geotactic climbing assays are a simple way of assessing the ‘neurotoxicity’ of compounds and mutations in Drosophila. The simplest of these assays involves recording the time taken for a certain number or percentage of a population of Drosophila to climb a given height after being knocked to the bottom of a tube. However, the stochastic nature of the geotactic behaviour introduces noise such that often many repeats are required for small effects to become statistically significant. High throughput assays have been implemented in various fashions including the simple procedure of using of multiple vials simultaneously and a digital camera to collect an image at a defined time point (Nichols et al., 2012). The most advanced high throughput screening system we have encountered is the automated tapper and image collection system (0.5 Hz) coupled to particle analysis (Podratz et al., 2013). We have developed a much simpler system which is based on a manual lever system for tapping and a webcam for video rate analysis of geotactic behaviour using image intensity at the bottom of the vial. This contrast technique does not require individual flies to be discriminated and can therefore report shorter latency responses when the local fly density is high.A plastic vial (50 mm high, 15 mm diameter) was loaded with ~12 flies, closed with a lid containing a dual gas port (syringe needle and vent hole) and was mounted using a circumferential clip to the end of a vertically pivoted 150 mm long lever arm (supported by retort stands) and imaged with a web-cam at a slightly downward angle (see Fig 1C). At 30 s intervals the vial was raised, by pivoting the lever arm (~10 degrees from vertical) to an end stop, and then allowed to fall back under gravity. Friction within the lever system was reduced such that the impact velocity of the vial with the plate was sufficient to knock all of the flies to the base (Fig 1C). The web-cam video for a sequence of such taps (~18) was recorded and exported for image analysis. Each tap was associated with a lower light intensity at the bottom of the vial as the dark flies obscured the light background. Using a custom made program, region of interest (ROI) measurements were made from the whole of the base of the vial and from a reference region of the image. The base ROI data was then corrected for gain changes of the web-cam using the reference ROI data. Then, using a custom set of macros, each tap was automatically detected and the minimum ROI value post tap extracted. Thus, it was possible to calculate a signal related to the percentage of flies on the base of the vial (Fig 1A). A comparison with manual counting reveals a good correlation (r2=0.92) between the percentage flies signal and integer fly numbers (Fig. 1C).Using this technique we have investigated the effect of various anaesthetic gases on geotactic behaviour. Fig. 1A shows the effect of 20% CO2 exposure followed by the later addition of 0.1% NH3. Since the technique allows for both high temporal and spatial resolution different elements of the geotactic response can be investigated including early latency responses. The simplicity of the technique makes it especially attractive for use in undergraduate projects. However, replacing the web-cam with a simple photodiode could further lower the cost making it suitable for undergraduate practical sessions.