An analysis of evoked responses of Purkinje cells in developing rat cerebellum to climbing fiber input was conducted to determine which identifying properties of this afferent system are established early in development and which specific features mature with age. Rat pups at various ages were anesthetized with 0.5% halothane and unit recordings made with glass micropipettes. By the third postnatal day, electrical stimulation of the sensorimotor cortex and limbs at low stimulation rates (less than 1/sec) could elicit distinct burst responses appearing at long latencies (180 msec), indicating that pathways of both ascending and descending climbing fiber systems are intact early in cerebellar cortical development. A distinctive feature maturing over the first 1-1.5 weeks was the characteristic of the all-or-none burst response since before about day 11 the interspike interval, amount of inactivation, and number of spikes in evoked burst responses all varied from stimulation to stimulation. Mean latencies decreased from 180 msec at dat 3 to 50 msec by day 10, but did not achieve the adult value of 20 msec until the fourth week. Typically, climbing fiber responses could only follow at stimulation rates of less than 0.2/sec at day 3, but by day 12 could follow up to 10/sec, which is the same as in the adult. The data indicate that the climbing fiber system establishes connectivity from diverse sources and exhibits identifying characteristics similar to the adult early in cerebellar development. Most aspects of the maturation of transmission can be explained if there were a decrease in the time scale of function of the synapses involved, mainly those in the inferior olive.