Measure markers of stress and in particular hypoxia taking into account the postmortem delay (PMD) in the sudden infant death syndrome (SIDS). We studied in children who died of SIDS different stress and PMD markers by immunoanalysis such as RIA/IRMA with groups of 30–50 SIDS compared to age paired controls ( n > 30). We chose (i) erythropoietin (EPO) which is regulated by tissue O 2 pressure, (ii) stress hormones: cortisol, ACTH, beta endorphin, adrenergic markers: catecholamines (by HLPC) and their co-factors i.e. neuropeptide Y (NPY) and Chromogranin A (CGA), (iii) the most reliable markers of the PMD to measure the variations of the biological markers according to the PMD. We studied K+, lactate/glucose ratio and NSE (neuron-specific enolase) in CSF. After establishing the physiological levels of EPO according to the age of the infants, we showed that there was a significant difference in the EPO of SIDS and age-matched control infants (Le Le Cam-Duchez V, et al., Biol Neonate 1999;76:1–9); pre-mortem anemia was excluded in SIDS because the fetal hemoglobin % was normal. At the same time, we have shown that SIDS have an increase in the CSF and the serum of the couple ACTH and beta-endorphin (BE) whose secretion is equimolar and of serum cortisol (M ± SEM: 874 ± 74 nM vs normal < 250 nM). Unlike ACTH, which is labile (CSF: 28 ± 7 pM; blood: 7 ± 3; normal < 15 pM), BE remains stable during postmortem time (CSF: 123 ± 21 pM; blood: 24 ± 9; normal < 15 pM). During agonal stress Cortisol and BE are both greatly increased signifying maximal stress (Coquerel A, et al. Neurochem Int 1992;20:97–102). For adrenergic markers, catecholamines cannot be interpreted because they are very labile; nevertheless, HPLC-MSMS makes it possible to characterize and quantify toxins and drugs. On the other hand, neuropeptide Y and especially Chromogranin A (CGA), which are co-released by the adrenal medulla, are stable in PMD (CGA: m ± SEM: 265 ± 22 vs. control infants 105 ± 18 ng/mL). They are greatly increased among the SIDS. As for the DPM markers for PM times > 12 h, the most reliable is the NSE. It also seems necessary to look for toxins and drugs in biological fluids because they can contribute to fatal apnea (Kahn A, et al. Biol Neonate 1994;65:235–39; Pathman L, et al. J Vascular Anom 2022;1–3). Most SIDS cases are characterized by stress markers related to long-lasting hypoxic episodes with EPO, cortisol + endorphin and CGA, which are quantitatively assayed up to 48 h postmortem and PMD could be specified by K+, Lactate and NSE in CSF.