INTRODUCTION Growth potential of the lambs is one of the most important traits in a genetic improvement scheme for meat sheep. A number of non-genetic factors affect these growth traits and directly obscure recognition of the genetic potential. Adjustment of data for non-genetic factors and estimation of genetic parameters for the various traits are necessary to obtain reliable estimates for important economic traits and to increase the accuracy of selection of breeding animals. An effective breeding plan can only be devised after thorough knowledge has been obtained about the inheritance of economically important traits. Estimates of heritability and genetic and phenotypic correlations form the basis of such information. This knowledge is required to formulate optimum breeding objectives and an effective genetic improvement programme. The Mecheri sheep is one among the recognised mutton breed of sheep in India and is widely distributed in semi-arid region of north-western part of Tamil Nadu namely Salem, Erode and parts of Dharmapuri and Namakkal districts. The Mecheri sheep is known for its relatively good growth rate (Ganesakale and Rathnasabapathy, 1973; Acharya, 1982), higher dressing percentage (Arumugam et al., 1978) and better skin quality (Karunanithi et al., 2005). The study on production performances of this breed over the years in a semi-intensive system of management, which is similar to the management of the flock by the farmers, is lacking. In addition, the effects of various factors such as period and season of birth, sex of the lamb and parity of the dam have not been studied. Hence, the present study was undertaken to identify various factors influencing the growth traits and to estimate the genetic and phenotypic parameters of these traits in Mecheri sheep. The information thus collected will be useful to identify the most important environmental factors influencing the genetic potential of the animals, which in turn will be useful for formulating a suitable breeding programme and genetic evaluation of the breed for implementing breed improvement programmes at the farmer and institutional levels.