Rosmarinic acid (RA) is a natural antioxidant that has many biological activities. In the present study we investigated the potential of RA to reverse the negative effects of the widely used antibiotic and antiprotozoal agent metronidazole (MTZ), which is known to induce reversible male infertility. Two doses of RA (5 and 15mg kg-1) were studied in sexually mature rats with and without MTZ-induced infertility. Rats were intraperitoneally injected with 5mg kg-1 RA or 15mg kg-1 RA (in distilled water) and, 45min later, they were intraperitoneally injected with 40mg kg-1 MTZ (in distilled water). Cauda epididymidal sperm suspensions were used to assess sperm count, motility and morphology. Histological and ultrastructural studies were performed on the testes and cauda epididymidis. In rats in which infertility was not induced, neither dose of RA affected the parameters assessed. However, in sexually mature rats in which infertility was induced by 40mg kg-1 MTZ, RA at both 5 and 15mg kg-1 ameliorated the damaging effects of MTZ on final bodyweight (30 days later), sperm motility and morphology. Only 5mg kg-1 RA, and not 15mg kg-1 RA, improved the harmful effects of MTZ on the sperm count and testis ultrastructure. The findings of the present study have considerable clinical implications and suggest a possible use for RA to reverse the negative effects of MTZ on male fertility, the male reproductive system and spermatogenesis.