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The male sand rat (Psammomys obesus), captured alive in the Sahara desert in the area of Beni-Abbes (Algeria), exhibited seasonal changes in plasma concentrations of LH, characterized by an increase in early summer. Administration of a standard dose of GnRH (200 ng/100 g body weight) failed to elicit significant season-dependent changes in LH release, whereas the increase in plasma testosterone was maximum in June-July and quite small between November and March-April. The present results suggest that the summer seasonal onset of the testicular endocrine activity of the sand rat is due to increases both in LH release and in testis sensitivity to gonadotropin.