In a randomized double-blind placebo-controlled trial involving 71 patients, a viscous gel containing distilled water or 1 or 2 mg pure porcine relaxin was instilled in the cervical canal on the evening before the surgical induction of labor. Eleven of 48 patients receiving relaxin labored overnight, whereas only one of 23 patients went into labor. Only the 2-mg dose significantly improved the mean cervical score compared with the placebo treatment; the effect was greatest in primigravid patients with unripe cervixes. Intracervical application appeared to confer no benefit over vaginal application in effecting cervical ripening or inducing labor. Systemic absorption of the porcine relaxin after its intracervical application was confirmed by the measurement of immunoreactive relaxin in a homologous porcine relaxin radioimmunoassay. Thus, the cervical ripening effect of exogenous relaxin may be mediated either systemically or by direct action at the site of local application. This trial confirms the responsiveness of the human term cervix to exogenous relaxin and supports the suggestion that endogenous relaxin may play a similar role at term in facilitating cervical ripening and parturition.