Presents the design and construction of sensitive circuits of a radiosonde for measuring the meteorological parameters of temperature, water-vapour pressure and atmospheric pressure in a tropical environment. The design includes the sensor, multiplexer and parameter-to-frequency conversion circuits. Conversion sensitivity of 3.3 Hz/C degrees was achieved for the temperature-to-frequency converters, employed in psychrometer mode to measure water vapour pressure. The sensitivities of the present circuits are much better than those obtainable with the Weather Measure Corporation's RD 1680 radiosonde, which has a conversion sensitivity of about 0.7 Hz/C degrees .