Fusion of membranes occurs in diverse biological events and, in most cases, Ca2+ greatly augments its rate. The aim of this work was to study the role played by Ca2+ when transplanting exogenous proteins into Xenopus oocyte membranes. Lipid vesicles carrying nicotinic acetylcholine (ACh) receptors (nAChRs) from Torpedo electroplaques were injected into oocytes. The time course of nAChR incorporation was assessed by recording ACh-evoked currents at different times from injection. An incorporation peak was found at 16 h, but responses were maintained for over 48 h. To assess the role played by Ca2+, two groups were considered: control and chelator-loaded oocytes. In the latter group, cells were incubated with 50 microM 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetrakis(acetoxymethyl)ester (BAPTA-AM) or loaded with ca. 5 nmol ethyleneglycol-bis (beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) 2 h before nAChR injection. Both groups responded to ACh, although the current amplitude was smaller in chelator-loaded than in control cells. These results indicate that the slow fusion of lipoproteosome vesicles with the oocyte plasma membrane does not depend on intracellular Ca2+ increase and therefore belongs to the type called "constitutive". This membrane fusion process is thus different from those involved in resealing of disrupted oocyte membranes or in the fusion of cortical granules with the egg membrane.