The suggestion by Land (1973) that it might be possible for genes with positive effects on female reproductive performance to be identified in males, has provided the stimulus for many studies in sheep populations throughout the world. The review of literature (Chapter 1) established, however, that these studies have largely been restricted to between-breed comparisons or within sheep populations which have been selected for components of reproduction rate. The review of literature also established that, although physiological and endocrine aspects of gonadal growth and function in rams has been extensively studied, relatively little research has been directed at sources of variation in male reproductive traits with a view to estimating genetic parameters. A similar situation was found when the literature relating to ram libido and serving capacity was reviewed. The aim of the studies described in Chapters 3 and 4 of this thesis was to identify, in a large random breeding Merino flock maintained at Trangie, NSW, the sources of variation in measures of gonadal growth and serving capacity, and to estimate genetic parameters for these traits. Utilising appropriate variances from these studies and other sources, an investigation (Chapter 5) was conducted, aimed at quantifying the genetic relationship between male and female measures of reproductive performance. The study described in Chapter 6 is the result of an investigation of a related aspect of male and female genetic relationships.