Green fluorescent protein (GFP) from Aequorea victoria was used as an in vivo reporter protein when fused to the C-terminus of the Jen1 lactate permease of Saccharomyces cerevisiae. The Jen1 protein tagged with GFP is a functional lactate transporter with a cellular abundance of 1670 molecules/cell, and a catalytic-centre activity of 123 s(-1). It is expressed and tagged to the plasma membrane under induction conditions. The factors involved in proper localization and turnover of Jen1p were revealed by expression of the Jen1p-GFP fusion protein in a set of strains bearing mutations in specific steps of the secretory and endocytic pathways. The chimaeric protein Jen1p-GFP is targeted to the plasma membrane via a Sec6-dependent process; upon treatment with glucose, it is endocytosed via END3 and targeted for degradation in the vacuole. Experiments performed in a Deltadoa4 mutant strain showed that ubiquitination is associated with the turnover of the permease.