The effects of the protease trypsin, externally applied to full-grown oocytes of Xenopus laevis, were studied using electrophysiology and fluorometry. The following results were obtained: trypsin in concentrations of 0.1 microgram/ml to 1 mg/ml liberated Ca2+ from internal stores and evoked large transient currents of up to 5 microA in bath solutions containing 1 mM or no Ca2+. The response desensitized for 50 minutes and recovered at longer times. Transient currents could also be elicited by tryptic impurities in commercially available collagenase used for defolliculation of oocytes. Application of chymotrypsin (0.01 or 1 mg/ml) or of thrombin (3.4 ng/ml or 0.34 mg/ml) neither evoked currents nor desensitized trypsin responses. Incubation with 1 microgram/ml Pertussis toxin for 20 to 25 hours prevented the Ca2+ release from internal stores and the activation of transient currents by trypsin. We propose that endogenous receptors in the oolemma, specific for trypsin, are linked to internal Ca2+ stores via Pertussis toxin-sensitive G proteins. Thus, receptor activation by external trypsin raises internal Ca2+ and thereby opens Ca(2+)-activated Cl channels in the oolemma.