The aim of the present study was to investigate the effects of hot summer climatic conditions (compared to winter) on the libido, semen quality traits, level of serum testosterone hormone and some physiological and body measurements of Egyptian Suffolk rams. A parameter for the tolerance of the rams to adverse hot climatic conditions, designated as the tunica dartos index (TDI), was estimated as the multiplication of the percentage change in scrotal length and that of the percentage difference between rectal temperature and scrotal skin temperature. The relationships between the TDI and semen characteristics were also estimated. The results showed that in summer, the reaction time, pH, percentage dead and abnormal spermatozoa, acrosomal damage and temperatures of rectum, body skin and scrotal skin and scrotal length were significantly higher, while sperm motility, sperm concentration, scrotal circumference and testis length were significantly lower than in winter. The correlation coefficient (r) values estimated between the TDI and each of the semen pH, ejaculate volume, sperm motility and sperm cell concentration were positive, and those estimated between the TDI and each of the reaction time, dead and abnormal spermatozoa and acrosomal damage were negative. The best values of libido (low reaction time), semen pH, semen motility percentage, sperm cell concentration and testosterone level were recorded with a high TDI level (7.5 and more), while those of percentage of dead spermatozoa, sperm abnormalities and acrosomal damage were recorded with a low TDI level (less than 4.5). It can be concluded that the TDI correlates well with the reproductive ability of the ram and can be a reliable parameter to indicate the tolerance of rams to hot subtropical conditions in Egypt.