The spontaneous arm movements of 28 healthy full-term newborn infants were observed for 1 min in four different environmental conditions: each infant was placed in a vertical position in a bathtub filled with water with the water level up to her or his neck in a vertical position in water with the water level up to her or his waist in a supine position out of the water in a vertical position out of the water. It was hypothesised that these different environmental constraints would influence the infants spontaneous activity. From video recordings spontaneous arm movements were categorised according to the classifications of Hannan (Hannan TE. Young infant's hand and finger expressions: An analysis of category reliability. In: Field T, Fogel A, editors. Emotion and Early Interaction. New Jersey: Elbaum 1982;253-265). The statistical analyses showed, firstly, that the frequency of occurrence of various arm movements is greater out of the water than in the water. Secondly, for those out of the water the frequency of occurrence of various arm movements is greater in the vertical position in comparison to the supine position. Thirdly, the infants showed more active arm movements (more upward movements) in the vertical position out of the water in comparison to the two water and supine conditions. These findings lead to the conclusion that different environmental constraints influenced the spontaneous arm movements. Copyright (C) 1999 Elsevier Science Ireland Ltd.