The development of the intraovarian adrenergic nervous system was investigated in the guinea-pig by use of chemical determination of catecholamines with high performance liquid chromatography (HPLC) and with the formaldehyde-induced fluorescence method for visualization of adrenergic nerves (Falck-Hillarp technique). Ovaries from fetuses (39-40, 45-50, 55-57, 60-63 days of gestation) and young animals (1, 2, 3, 7, 14, 30, 40-45 days of age) were included in the study. The noradrenaline concentration was low in the ovaries from the youngest fetuses but increased with age, reaching a maximum level at 2 days post partum. A marked decrease in noradrenaline concentration from the second to the third day of life was found as a consequence of the rapid increase in the ovarian weight during this time. A similar decrease in ovarian noradrenaline concentration after a period of rapid ovarian growth was noted at 30 days of age. Measurable amounts of adrenaline were found in the ovary only in the fetal stages; the highest concentration (0.73 microgram) was detected at 55-57 days of gestation.