RESUMEN
Given the importance of the hormones prolactin and alpha-melanotrophin in the regulation of the mammalian reproductive cycle, it was decided to assess their serum levels during the reproductive cycle of Columba livia (domestic pigeon). Female pigeons were utilized in the stages of egg-laying, incubation and rearing. Simultaneously, the structural changes of the ovary in the aforementioned periods were studied. The levels of prolactin and alpha-melanotrophin showed similar behaviour. The low concentrations found in the period of egg-laying (x +/- s = 4.3 +/- 0.6; 3.5 +/- 0.26 ng/ml) increased significantly during incubation (x +/- s = 6.5 +/- 0.7 ng/ml; 6.6 +/- 1.7 ng/ml) to reach the maximum value in the rearing period (x +/- s = 10.33 +/- 1.8 ng/ml; 13.6 +/- 1.9 ng/ml). Accompanying these changes a marked predominance of the medullary zone over the cortex and lack of developing follicles were found in the rearing state.