Authors studied phenobarbital level's in serum in 21 newborns with convulsant syndrome. They consider serum's levels after five minutes and after 12-24 hours, the best time to study maximum action and duration of levels in relation to sedative limits. Non-radio-active immunoanalysis was used as markers instead reactive substances of enzymes. Enzymatic activities are measured by addition of substrates for enzyme employed (in this case G-6-PDH). PB dose employed was between 10 mg/kg IV in nine newborns and 20 mg/kg in another 13. All of them experienced a support dose between 2 and 4 mg/kg/24 hours during a week's time after sedation. Variable affects were: only 6 cases (28%) followed a good evolution with PB only. Two anticonvulsant drugs were used in 11 (52.4%) and 4 (19%) needed one more. They don't reach any relation between etiology of convulsant syndrome, PB doses and serum's levels.