To test the hypothesis that prolactin (PRL) plays a role in the hormonal events of labor, serum PRL levels in 15 normal secundigravidas were measured on 2 occasions 10–15 days before delivery, at the onset of labor, at cervical dilatation of 5 and 10 cm, at the time of delivery, and on the first, second, and fifth days postpartum. The mean level of PRL was 163 ng/ml ± 26 ng/ml at the onset of cervical dilatation; it typically decreased with the progress of labor, reaching a value of 140 ng/ml ± 21 ng/ml at the time of delivery. The differences during the various stages of labor, however, were not found to be statistically significant. Postpartum values were significantly lower (P < 0.01) on the fifth day after parturition. It is therefore unlikely that PRL is involved in or influenced by the hormonal interplay that occurs during labor.