The effect of exogenous prolactin on prolactin receptor (PRLR) gene expression and wool growth in pregnant and non-pregnant Romney ewes was assessed. Three experiments were performed where exogenous prolactin was administered by subcutaneous injection (daily for 18 days) or constant infusions (for 3, 9 or 18 days) and endogenous prolactin secretion was altered by exposing ewes to long day or short day photoperiods. Prolactin administration started a week after mating (in autumn), or in non pregnant ewes in mid-spring. Blood samples were collected for measurement of circulating prolactin by radioimmunoassay, skin biopsies were collected for the quantification of PRLR long (PRLR-L) and PRLR short form (PRLR-S) mRNA expression using real-time PCR assay. Wool patch samples were clipped monthly for assessing wool growth. Constant prolactin infusion of more than 3 days activated a positive feedback mechanism for PRLR-L synthesis, resulting in a sustained elevation PRLR-L mRNA expression for up to 38 days after infusion was over. This was associated with short- and long-term stimulation of wool growth in the pregnant Romney ewe. The main increase in wool production happened after parturition. This positive effect on wool growth by prolactin treatment was related to the length of prolactin treatment. A 3 day infusion resulted in a smaller degree of enhancement compared to the 9 days and 18 days. The biggest impact on wool growth was observed in one of the 18 days infused group, which resulted in a 25% increase in clean fibre production when compared to the pregnant group. The expression of PRLR-S mRNA was not associated with an elevation of prolactin levels. Daily injections neither increased PRLR-L mRNA expression nor increased wool growth, demonstrating that a constant and moderate increase in prolactin levels is necessary to stimulate PRLR synthesis. Data obtained in these trials also suggests that other reproductive hormones may influence PRLR expression and wool growth. The non-pregnant groups showed steady levels of PRLR-L mRNA expression, which could be associated with changes in hormonal levels due to the reproductive cycle. Seasonal molecules could also interfere with the system, as prolactin manipulation in non-pregnant ewes exposed to an artificial short day environment during spring time showed a different pattern of PRLR-L and PRLR-S mRNA expression and no wool growth effect. A mathematical model of prolactin/PRLR interaction was shown to be a good predictor of short-term PRLR gene expression, as its simulations agreed with our biological data. However, the inclusion of other gestational and seasonal hormones may be necessary if the model is to be used for simulations of long-term PRLR expression and wool growth during pregnancy and lactation. Overall, these results suggest that seasonal wool growth can be manipulated via prolactin, which increases PRLR-L mRNA expression resulting in enhancement of wool growth. However, there is a minimum period of constant prolactin elevation necessary to activate this positive feedback mechanism, Also there is a window of opportunity where this mechanism can be manipulated. This window is most likely associated with the animals interpretation of photoperiod, which also regulates the reproductive seasonality and therefore, could as well interact with prolactin in the regulation of PRLR mRNA expression and seasonal wool growth. This observation could lead to the development of products, suitable for on farm conditions, to enhance wool production.