INTRODUCTION: It has been described varied definitions of premature ejaculation (PE), which has determined different prevalences and rates of success for the different therapies with selective serotonin re-uptake inhibitors. Our goal was evaluate the effectiveness of paroxetine like treatment of premature ejaculation administered on-demand (4-6 hours previous to intercourse) compared to the scheme of daily dose. PATIENTS AND METHOD: A prospective study type crossover was designed with 14 patients. Grupo A: 7 patient received paroxetine 20 mg/d by three weeks followed by paroxetine 20 mg 4-6 hours before the intercourse by three weeks. Group B: the other 7 patients received the same scheme but replacing by placebo. Later to three weeks of therapy suspension, crossover was made. RESULTS: The intravaginal ejaculatory latency time (IELT) pre-treatment was 0.4 minutes. In the group A the IELT average was of 4.3 minutes in the treatment with daily paroxetine; 5.8 minutes when they received paroxetine on-demanad; 0.9 with daily placebo and 0.6 with placebo on-demand (p < 0.001). For group B the IELT during the daily placebo was 0.8 minutes and with placebo on-demand it was of 1.1. When they received daily paroxetine the IELT was 3.3 minutes and during the phase of paroxetine on-demand it was increased to 6.1 (p<0.001). CONCLUSIONS: The treatment of premature ejaculation with paroxetine in daily dose and scheme on-demand appears similar like effective options.