SUMMARYPatterns of circulating prolactin bioactivity during three stages of the menstrual cycle were studied in detail in four normally cycling women. Serial blood samples were withdrawn at 10 or 15 min intervals for up to 8 h. Plasma prolaction concentrations were quantitated by a bioassay which is based on the replication of a rat lymphoma cell line (Nb2node) and by conventional radioimmunoassay. A total of 50–103 samples from each patient was analysed. Using a threshold‐based algorithm, definite pulsations of prolactin biological (B) and immunological (I) activity were detected at most stages of the menstrual cycle. The periodicity of these peaks ranges from 1 to 11/2 h. However, pulsations were less evident in one subject who had asymptomatic hyperprolactinaemia although circulating prolactin was fully bioactive (mean B:I ratios being 1 02–1.25). B:I ratios in the three normoprolactinaemic women were generally lower, ranging from 0.39±0.02 to 0–93±0.03. Elevations in the B:I ratio of normoprolactinaemic women were associated with bioactive peaks; this was not always the case in the hyperprolactinaemic subject. Thus, the lack of symptoms in this hyperprolactinaemic subject could not be due to bioinactive prolactin, but might perhaps be due to the absence of episodic prolactin secretion which would desensitise targ