A whole nerve-muscle preparation (about 160 mg) or an end-plate preparation (about 25 mg) of the rat phrenic nerve-hemidiaphragm were incubated with [3H]choline, to investigate choline uptake and choline metabolism. Choline uptake was measured from the disappearance of choline from the incubation medium during the loading period and from the retention of tritium in the tissue after the loading and washout period. Based on the results obtained with both methods the end-plate preparation takes up three times as much choline than the whole nerve-muscle preparation or a small muscle strip that was cut outside the end-plate region and had a similar size as the end-plate preparation. Choline uptake was not markedly affected by the degree of nerve activity or by a chronic denervation. However, hemicholinium-3 significantly reduced (50%) the choline uptake by the end-plate preparation. Most of the choline (70-88%) taken up was metabolized and incorporated into membrane structures. Phosphatidylcholine was the predominant metabolite in both preparations. The ratio of phosphatidylcholine/lysophosphatidylcholine in the end-plate preparation (16) was significantly lower than in the whole nerve-muscle preparation (31). This might indicate a higher metabolism of phosphatidylcholine in the end-plate preparation. It is suggested that choline uptake by the rat phrenic nerve-hemidiaphragm occurs mainly by the muscle fibres. The innervated part of the muscle fibres can accumulate more choline than the peripheral part outside the end-plate region, probably because of a very active choline phospholipid metabolism within the end-plate region.