In this study, the efficacy of metronidazole as a local spermicidal agent was investigated. The drug was formulated in a concentration of 5% as an acid gel for vaginal application as a local contraception. The minimum spermicidal concentration of metronidazole was evaluated in-vitro and found to be 0.2% w/v. The formulated gel showed instant immobilization and death of all sperm within 30 s. A clinical study was conducted to determine the drug concentrations in vaginal secretions of healthy women and women with symptoms suggesting genital tract infection every 15 min for a 1-h period after application of 1 g of the gel in the vagina. Drug concentrations in the infected group were significantly lower (P < 0.05) than those in the healthy group at all time intervals. The drug concentrations in vaginal secretions after 1 h of vaginal medication remained above the determined minimum spermicidal concentration (0.2%) in the two groups. Measurement of vaginal pH before and after medication revealed a significant drop (P < 0.05) in pH to its normal value in both healthy and infected groups as a result of gel application. In conclusion, the designed gel has potential advantages of achieving a long retention time and effective drug concentrations in the vagina for at least 1 h after application, and of maintaining vaginal pH within its normal range.