The objective of the study was to record how somatostatin levels in plasma are altered in response to breast-feeding during the lactation period and to relate somatostatin levels to the success of the lactational performance and to smoking habits. Fifty-two women were investigated 4 days post partum and 3-4 months later. Blood samples were collected and the levels of somatostatin-like immunoreactivity (below referred to as SLI) were measured with radioimmunoassay. The periods of exclusive breast-feeding and of mixed feeding were assessed as well as the milk yield. Smoking habits were noted. SLI levels were found to be significantly lower on day 4 after delivery, compared to 3-4 months later. Also the type of response to breast-feeding was different. Thus, a significant fall of SLI was seen during breast-feeding at the maternity unit, but not 3-4 months later. Smoking women breast-fed fully for a significantly shorter time than nonsmokers and had significantly higher SLI levels at onset of breast-feeding day 4 post partum. Whether the high somatostatin levels recorded in connection with breast-feeding in smokers are related to the shorter period of breast-feeding seen in this group remains to be established. In addition, the highest levels of somatostatin were seen the day after the very last breast-feeding and a possible role for somatostatin in the weaning process should be explored.