Retinas from mice of the C57BL/6 strain were sampled at frequent intervals from birth to postnatal day 33 to determine the numerical density of conventional and ribbon synapses within the inner plexiform layer (IPL) as a function of time. Synaptic arrays of the IPL were formed in three phases. During Phase I, from day 3 to day 10, conventional synapses were produced at a mean rate of 0.44 synapses/1,000 micrometer3/hour, but no ribbons were seen. During Phase II, from day 11 to day 15, ribbons formed at a rate of 0.38 ribbons/1,000 micrometer3/hour and conventional synapses were produced at a rate of 1.15 synapses/1,000 micrometer3/hour. Phase III began at day 15, the approximate time of eye opening in these animals, and was characterized by a sharp reduction in the rate of production of both ribbons and conventional synapses. During this phase ribbons achieved a final mean density of 113 ribbons/1,000 micrometer3 and conventionals achieved a final mean density of 250 synapses/1,000 micrometer3. Serial appeared in Phase II but remained at low densities.