The export of about 2600 Friesian heifers to Kenya between 1971-1973 provided an opportunity of studying the adaptation problems experienced by these heifers. Within the framework of the NUFFIC (Netherlands Universities Foundation for International Cooperation) a research project entitled 'Adaptation of Dutch Friesian heifers in the Tropics, with special reference to Kenya' was initiated. This project studied aspects such as health parameters, reproduction and growth in the imported Friesians. Under the aegis of the project we studied the changes in cattle hair coat characteristics. This topic was chosen for 3 main reasons: - The characteristics and the cyclic changes of the hair coat are important in the ecology of cattle. - Various authors have drawn attention to the importance of hair coat type as an indicator of the ability of beef cattle to adapt to tropical climates. - There were no data on adaptation problems, with regard to hair coats, of dairy cattle. To complement the Kenyan data, I studied hair coat characteristics in Friesian heifers in the Netherlands. By thoroughly reviewing existing literature on hair follicles, sweat glands and hair coat characteristics in cattle, I obtained data that could be compared with the data I collected in Kenya and the Netherlands. The literature review (Chapter 2.) comprises a résumé of the anatomy and physiology of the hair follicle and sweat gland (section 2.1.4.); a summary of hair follicle and sweat gland characteristics (section 2.2.12.), and hair coat characteristics of cattle (section 2.3.13.); a summary of coat shedding, the effects of photoperiod, temperature and nutrition (section 2.4.4.); and the major conclusions on how hair follicle, sweat gland and hair coat characteristics relate to heat exchange, performance and adaptation (section 2.5.5.). Chapter 3 describes the experiments. The methods of measuring hair coat characteristics are given in paragraph 3.1. The hair samples were measured for medullation, hair diameter, hair length, melanin content, hair ...