Oestrogen was measured in urine samples collected from captive females representing 7 species of New World monkey to provide an overview of the applicability of such formation in the noninvasive monitoring of ovarian function and to assess the potential applicability of such information in phylogenetic studies. Species available for study were the pygmy marmoset, common marmoset, red-bellied tamarin, cotton-top tamarin, golden lion tamarin, Goeldi"s monkey and the owl monkey. Oestrone conjugates were measured in serially collected urine samples to demonstrate ovarian cyclicity. Urine samples obtained during the luteal phase were subjected to HPLC to identify immunoreactive oestrogens; oestrone and oestradiol-17 beta accounted for almost all of the immunoreactive oestrogen detected while oestriol content was negligible. Urine samples obtained during the follicular phase and luteal phase were subjected to glucuronidase hydrolysis, sulphatase hydrolysis and acid solvolysis, which revealed that the major immunoreactive oestrogen metabolite was: (1) oestradiol sulphate in the pygmy marmoset and common marmoset, (2) residual oestradiol in the red-bellied tamarin, (3) residual oestradiol and oestrone glucuronide in the cotton-top tamarin, and (4) oestrone glucuronide in the golden lion tamarin, Goeldi"s monkey and owl monkey. A phylogenetic tree based on the above shifts in oestrogen excretion suggested that clawed New World monkeys are specialized and that the lineages leading to the study species split off in the following order: Goeldi"s monkey, golden lion tamarin, cotton-top tamarin, red-bellied tamarin, common marmoset and pygmy marmoset.