There is a need for a more reliable, non-invasive and alternative measurement site for the monitoring of arterial blood oxygen saturation in critically ill neonates when peripheral perfusion is poor. The anterior fontanelle, a unique anatomical feature of the neonate, has been presented as an alternative site for the estimation of arterial blood oxygen saturation (SpO(2)). A new fontanelle photoplethysmographic sensor and processing system has been developed to investigate fontanelle photoplethysmographic (PPG) signals and estimate SpO(2) values at this anatomical location. Preliminary clinical trials have shown that good quality PPG signals with large amplitudes and high signal to noise ratio can be obtained from the neonatal fontanelle. The estimation of SpO(2) values from the fontanelle were in broad agreement with a commercial foot pulse oximeter.