Sociosexual behavior was monitored on a daily basis for 3 months in 5 pairs of golden lion tamarins (Leontopithecus rosalia). Urine samples were collected daily from each female and urinary estrogen cycles were determined by radioimmunoassay. Mounts and copulations were observed during all phases of the estrogen cycle. Peaks or regular cycles in sexual behavior were not documented. There were no significant changes in affiliative behavior by females or males that were associated with changes in urinary estrogen values. A negative relationship between pair bond duration and frequency of sexual interactions was observed: newly established pairs exhibited 2-6 times more frequent sexual behavior than a long-established pair. The lack of a conspicuous sexual signal in female golden lion tamarins may be related to a pattern of continuous sexual receptivity. Both reproductive patterns, concealed estrus and continuous receptivity, are explicable in relation to either monogamous or polyandrous mating systems.