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The severe environments where Phymaturus lizards inhabit in the Andes highlands and in Patagonia, Argentina, impose restrictions on their reproduction, offering a framework for the development of life history strategies to overcome hard weather conditions. Among them, prolonged female cycles, asynchrony between sexes in receptivity, and sperm storage in males, were described. Asynchrony in the reproductive timing between males and females is a consequence of different energy requirements for gametogenesis, and often imply the existence of cellular mechanisms to enhance fertilization, such as the asynchronic steroid synthesis between testicular compartments, allowing gametogenesis independently of mating. In the present study ultrastructural and hormone assays were combined for the first time in liolaemids. Specifically, morphological features of steroid activity in Leydig and Sertoli cells, and serum testosterone concentrations have been studied in the lizard Phymaturus antofagastensis. Leydig and Sertoli cells presented morphological features characteristic of steroid synthesis during the spermatogenesis, and evident asynchronic steroid production between testicular compartments. Active Sertoli cells and inactive Leydig cells were observed in spring and autumn, while in mid-summer their steroid activity was synchronic in coincidence with maximal abundance of spermatozoa in epididymis. Serum testosterone concentration was at its maximum in mid-summer (126-230 ng ml(-1)), and minimum in late spring (4-24 ng ml(-1)) and early autumn (2-17 ng ml(-1)). In view of these results, P. antofagastensis males show an original approach to adjust their reproductive activity to physiological and environmental constraints at high latitudes and altitudes in the Andean highlands of Argentina., (Published by Elsevier Inc.)