Despite of intensive efforts, ethiopathogenesis of rheumatoid arthritis (RA) has not been definitely solved yet. Prolactin (PRL) has been found to affect immunological system which lead to speculation about its role in ethiopathogenesis of RA. The increased levels of PRL were found in RA patients in a majority of cases. The aim of this study was to investigate diurnal secretion of prolactin in patients with early onset of RA. Patients and methods. Ten patients were evaluated, 8 women with average age of 45.5 years and 2 men with average age of 34.7 years. Duration of the disease: 2.07 months. As controls we evaluated 6 volunteers, women with average age of 41.3 years. Blood was collected at 4 hours intervals. PRL was analysed by chemiluminiscence- immunochemical reaction with mono-clonal antibody using Imunolite. Results. Compared to the control group, higher levels of PRL were found in RA patients. Mean levels of PRL in the group of RA patients: at 8.00 a.m.: 9.52 ng/ml, at 12.00 p.m.: 9.71 ng/ml, at 4.00 p.m.: 12.21 ng/ml, at 8.00 p.m.: 14.11 ng/ml, at 12.00 a.m.: 18.95 ng/ml, at 4.00 a.m.:18,24 ng/ml. Statistically significant differences between control group and RA patients were found at 4.00 p.m. (*p < 0.05) and at 8.00 p.m., 12.00 a.m. and 4.00 a.m. (**p < 0,01). If we took the lowest cortisol levels measured at these times into account reciprocal ratio of prolactin/cortisol might then be involved in alteration of daily activity of the disease with maximum of difficulties in the morning. Better understanding to interactions between neuroendocrine and immune systems will help to reveal the ethiology of autoimmune disease and will contribute to search for new therapeutical strategies in the future.

        Key words: rheumatoid arthritis, prolactin, diurnal variation