Chloroplast replication has been studied in discs cut from the base of young spinach leaves and cultured on sterile nutrient agar. In discs grown in a growth cabinet chloroplast numbers per cell increased logarithmically with time over a 7-day culture period. Chloroplast replication proceeds in a similar way in cultured discs and in intact leaves. Cytokinins do not affect chloroplast replication in this system but they stimulate the fresh-weight growth of discs. Chloroplast replication is temperature dependent, having an optimum at 25 °C. By contrast chloroplast size is at a maximum in discs cultured at 12 °C. Light stimulates chloroplast replication, a linear relationship occurring between chloroplast number per cell and the daily quantity of light given to discs up to a saturating value of 250 J d−1. Daylength does not affect chloroplast formation in spinach. In a number of experiments a general relationship was established between chloroplast number per cell and cell size but no evidence is available to suggest that this correlation is causal. The results of experiments in which discs were transferred from dark to light suggest that some of the events which precede chloroplast replication may occur at similar rates in both light and dark.