Patch clamp studies show that there may be as many as seven different channel types in the plasma membrane of protoplasts derived from young leaves of the halphytic angiosperm Zostera muelleri. In wholecell preparations, both outward and inward rectifying currents that activate in a time- and voltage-dependent manner are observed as the membrane is either depolarized or hyperpolarized. Current voltage plots of the tail currents indicate that both currents are carried by K+. The channels responsible for the outward currents have a unit conductance of approximately 70 pS and are five times more permeable to K+ than to Na+. In outside-out patches we have identified a stretch-activated channel with a conductance of 100 pS and a channel that inwardly rectifies with a conductance of 6 pS. The reversal potentials of these channels indicate a significant permeability to K+. In addition, the plasma membrane contains a much larger K+ channel with a conductance of 300 pS. Single channel recordings also indicate the existence of two Cl- channels, with conductances of 20 and 80 pS with distinct substates. The membrane potential difference of perfused protoplasts showed rapid action potentials of up to 50 mV from the resting level. The frequency of these action potentials increased as the external osmolarity was decreased. The action potentials disappeared with the addition of Gd3+, an effect that is reversible upon washout.