The aim of the study was to evaluate the influence of vaginal douching with sodium bicarbonate (NaHCO3) upon cervical mucus viscoelasticity and sperm penetration in vitro and in vivo. Twenty-five couples with primary infertility for greater than 12 months participated in the study. The selection criteria were: (i) semen quality compatible with conception, (ii) regular ovulatory cycles and (iii) repeated negative post-coital test (PCT). After at least one inventory cycle, three consecutive cycles were studied. In the second and third cycles, vaginal douching was performed with either 1.5% (w/v) NaHCO3 or 0.9% (w/v) NaCl (randomized procedure). The viscoelasticity of the cervical mucus, sperm penetration tests (SPT) and PCTs were analysed. The viscoelasticity of mucus samples after NaHCO3 douching was significantly lower than the viscosity after NaCl douching (P less than 0.001, n = 16) and in the control cycles (P = 0.003). The SPT scores were significantly higher in the NaHCO3 cycles than in the NaCl cycles (P = 0.004, n = 22) and in the control cycles (P less than 0.001). The PCT scores proved to be significantly higher after NaHCO3 douching than after NaCl douching (P = 0.002, n = 21). Comparison of NaHCO3 and control cycles also showed a significant improvement of the PCT score after NaHCO3 douching (P less than 0.001).