We present a method for non-invasive electrophysiological analysis of rapid escape responses in intact, freely behaving larval medaka (Oryzias latipes) before and after short-term exposure to environmental toxicants. Recordings are obtained as a larval medaka swims in a small chamber of water above pairs of recording electrodes etched onto a printed circuit board. When the chamber is briefly vibrated by a sinusoidal pulse from a speaker attached to the grid, a stereotyped Mauthner cell-initiated escape response is evoked. The following parameters were quantified from recordings: 1) Mauthner axon conduction velocity, 2) delay between Mauthner axon spike and motoneuron spike in the spinal cord, 3) latency from stimulus onset to Mauthner spike, and 4) percent of success (or failure) of Mauthner responses during a series of stimuli. Toxicant-induced changes in these parameters were described following exposure to phenol, which tended to increase central delay, onset latency, and percent of response failure. This contrasts with effects of strychnine, which tended to decrease central delay and onset latency.