A new in situ incubation technique was developed to measure primary production in sea ice. This method allows fine-scale measurements in 1 cm thick vertical sections throughout the ice column without severe disruption of ice morphology, geochemistry and light field. Easy handling in the field makes this device usable for all scientists working on sea ice biota. Primary production within first(FYSI) and multi-year sea-ice (MYSI) cores of the Barents and Greenland Seas was determined during the RV 'Polarstern' cruise ARK XIII/1(a+b). In order to measure algal production, 6 slices of 1 cm thickness were cut off at regular intervals along an ice core and incubated in sealed glass petri dishes. The incubation chambers and a chamber for dark fixation, together with the remaining sections of the ice core, were placed in an acrylic-glass barrel of 1 m length, and returned to their original positions inside the core hole for in situ rate determinations. Additional ice cores were drilled to determine ice temperature and nutrient and algal pigment concentrations. The incident photosynthetically active radiation (PAR) as well as under ice irradiance was measured. Steep gradients in primary production were found throughout the ice floes, with maximum values of 7.7 µg C l-1 h-1 in the bottom few centimetres corresponding to the highest concentrations of chlorophyll a (84 µg l-1). Nutrient concentrations in brine varied between ice stations and with ice depth: 0-88.4 µmol l-1 for NH4; 2.2-13.6 µmol l-1 for NO3; 0-0.5 µmol l-1 for NO2; 0-12.4 µmol l-1 for PO4; and 0.2-23.5 µmol l-1 for SiO4. Mean integrated production at the 4 stations (0.76-9.67 mg C m-2 d-1) varied with PAR and total dissolved nitrogen. Integrated production of the ice interior community (more than 5 cm from bottom of floe) was in the same range as or sometimes exceeded production in the bottom community (0-5 cm) of FYSI as well as MYSI. Growth rates of the ice algae ranged from 0.01 to 0.5 d-1. As previous primary production methods were restricted to the study of bottom communities only, we assert that algal primary production in Arctic as well as Antarctic sea ice has been severely underestimated.