-Objective: To evaluate the influence of gestational age (GA) and birth weight (BW) in the clinical assessment of the muscle tone of healthy term and pre t e rm newborn s . Method: C ross sectional study. The muscle tone of healthy 42 pre t e rm and 47 term newborns was quantified and measured with a goniometer (an instrument for measuring angles) respectively between 7 th -14 th day of life and 24-48 hours of life. Newborns were grouped according to GA and BW and evaluated at fixed time intervals by one examiner. P re t e rmnewborns were matched to term at 40 weeks postconceptional age (PCA). Results: The evolution of muscle tone in the pre t e rm occurred gradually, following PCA, independent of birth weight. Pre t e rm n e w b o rns had lower scores in all muscle tone indicators when compared to term at the first assessment. D i ff e rences were observed among pre t e rm small for GA and adequate to GA for the indicator heel to ear (p<0.001). When compared at 40 weeks PCA, except for posture, all other indicators were significantly diff e rent (p< 0.001) among gro u p s . Conclusion: P rematurity and intrauterine malnutrition are influential factors in some indicators of the newborn muscle tone state. Muscle tone assessment of pre t e rm infants does not seem to be influenced by birth weight, however evolution is clearly related to postconceptional age. When we compared term newborns AGA and SGA it seems to have a clear influence of the birt h weight on some indicators of the muscle tone. The use of devices such as the goniometer allows the perf o rmance of a more objective assessment of muscle tone and helps to quantify findings.KEY WORDS: preterm newborns, muscle tone, low birth weight, intrauterine malnutrition.A influência da idade gestacional e do peso ao nascimento na avaliação clínica do tono muscular de recém nascidos a termo e prematuros hígidos RESUMO -O b j e t i v o :Verificar a i n f l u ê n c i a da idade gestacional (IG) e do peso ao nascimento (PN) na avaliação do tono muscular de recém nascidos (RN) a termo e pre m a t u ros hígidos. Método: Estudo transvers a l . O tono muscular de 42 RN pre m a t u ros e 47 termo foi quantificado e mensurado com goniômetro (instru m e nto para medir ângulos), respectivamente entre 7-14 dias de vida e 24-48 horas de vida. Os RN foram agru p ados de acordo com IG e PN, sendo avaliados em intervalos fixos. RN prematuros foram comparados aos a t e rmo na 40ª semana de idade concepcional (IC). Resultados: A evolução do tono muscular nos RN pre m at u ros ocorreu de forma gradual de acordo com IC e independente do PN. RN pre m a t u ros, na primeira avaliação apresentaram escores de tono muscular inferiores aos de RN a termo em todos os indicadore s . Foram observadas diferenças entre RN pre m a t u ros pequenos e adequados para a IG em relação ao indicador calcanhar-o relha (p<0,001). O pareamento dos grupos na 40ª semana de IC mostra, exceto pela postura, diferença significativa entre os grupos (p< 0,001) nos demais indicadores. Conclusão: A prematurid...