Ten women were studied during the 1st post-partum week. Suckling for 20 min induced a marked increase in plasma prolactin [PRL], reaching a maximum within 0-25 min after the end of suckling and then returning to pre-suckling levels after 120 min. Suckling induced no changes in plasma thyrotropin (TSH), growth hormone (GH) or follicle stimulating hormone (FSH). The i.v. injection of 200 .mu.g of thyrotropin-releasing hormone (TRH) immediately after suckling resulted in an additional increase in plasma PRL and a rise in TSH. When given 120 min after suckling, TRH was followed by increased plasma PRL and TSH levels, which for both hormones were of a magnitude comparable to the TRH induced increment seen immediately after suckling. Suckling did not inhibit the effect of TRH on the release of TSH. TRH is probably not involved in the suckling induced increase in PRL secretion. The mean plasma FSH level was below the limit of detection before and after suckling. Neither plasma FSH nor PRL showed any apparent changes following the i.v. injection of 25 .mu.g of luteinizing hormone-releasing hormone (LH-RH), when given immediately after and 120 min after suckling. When given after suckling as indicated above, TRH induced no changes in plasma GH or FSH and similarly LH-RH was without influence on plasma GH and TSH.