Amphibian activity is affected by environmental variables, and in some species from tropical and subtropical regions the males in activity are bigger at the beginning of the rainy season than at the end. In this context, the relationship between male calling activity and climatic variables was evaluated in a population of Boana pulchella from Uruguay. This tree frog is distributed from southern Brazil to central and northern Argentina and is a prolonged breeder, enabling us to evaluate temporal male body size variations. The study population was in a temperate region with seasonal variation of temperature and aseasonal rainfall. Monthly mean temperatures and total rainfall were obtained from a meteorological station 15 km from the semi-permanent pond where the sampling work was conducted. From November 2005 to November 2006, 10 m from this pond, and three days per month, air temperature and relative humidity were recorded, and monthly photoperiods were calculated. From 20:00–22:00 on the days climatic variables were determined, the activity of B. pulchella was measured through an acoustic and visual encounter survey. In addition, animals were collected, measured, and released. Male activity was not correlated with either mean temperature or photoperiod. This was compatible with the aseasonal male activity calculated with circular statistics. However, male activity was positively correlated with rainfall. The occurrence of froglets was seasonal, concentrated mainly in November 2005. The males in the cold season were larger than those in the warm season. Qualitative comparison among calling phonologies at different latitudes indicated B. pulchella was inactive during some cold months in the south and during some warm months in the north. This geographical variation in calling phenology indicates that local environmental temperatures affect activity and likely reproduction of B. pulchella, with the optimum being in the range of cold to mild.