Venipuncture for blood collection is postulated to be a stress factor that can increase circulating prolactin (PRL) and therefore produce false results. Seven normal non-pregnant women and 44 gravidae were submitted to venipuncture with a butterfly needle connected to a syringe for vein maintenance. Blood was collected immediately (zero time) and 15 and 30 min after venipuncture. After the last collection, a vein in the contralateral arm was punctured in the non-pregnant women only and a new blood sample was collected. PRL, measured by immunoassay, varied from 7 to 13.2 ng/ml in the non-pregnant women and from 7 to 300 ng/ml in the gravidae. No significant difference in PRL values was detected at any of the collection times. It is concluded that the stress or pain caused by venipuncture is not sufficient to alter plasma PRL levels in normal (non-pregnant) or hyperprolactinemic (gravidae) women.