Isoprene emission from vegetation is the single most important source of photochemically active reduced compounds to the atmosphere. We present the first controlled-environment measurements of isoprene emission from leaves of tropical forest trees. Our studies were conducted in the Guanica State Forest in Puerto Rico. We report the effects of temperature and light variations on biogenic isoprene emissions during 1995. Maximum emission rates varied among species from 0 to 268 nmol m−2 s−1. Values at the upper end of this range of maximum emission rates are 2–3 times higher than values reported from any temperate taxa. Isoprene emission showed strong sensitivity to light and temperature variations. In contrast to temperate plants, whose emissions tend to saturate at a light intensity of ∼1000 μmol m−2 s−1, emissions from the tropical species increased with light intensity up to 2500 μmol m−2 s−1. The temperature optima for emissions from these plants were similar to those previously reported for temperate plants: ∼40 °C. The high maximum emission rates and lack of light saturation indicate that estimates of isoprene emission from tropical forests need to be revised upwards.