During prolonged exposure to extracellular 4-aminopyridine (4 AP) the periodic activity of the somatic membrane of an identified molluscan neurone passes from a repetitive regular discharge of greater than 90 mV amplitude action potentials, through double discharges to less than 50 mV amplitude oscillations. Return to standard saline causes the growth of parabolic amplitude-modulated oscillations that develop, through chaotic amplitude-modulated oscillations, into regular oscillations. These effects are interpreted in terms of the actions of 4 AP on the dynamics of the membrane excitation equations.