A procedure is described for prolonged sampling of blood from unrestrained immature rats which overcomes some of the problems associated with contemporary techniques. 1 or 2 days before sampling, the animal is fitted with two indwelling catheters, one inserted into the right jugular vein and the other into a femoral vein. On the day of the experiment blood is continuously withdrawn from the jugular vein (30 µl/min) using a peristaltic pump and is dispensed into sample tubes every 5 min by means of an automatic fraction collector. A second channel of the same pump continuously infuses a blood replacement mixture (at 37 °C) into the femoral vein thereby keeping the animal’s blood volume constant. In the present study this technique was used to sample blood from both ovariectomized and immature, intact female rats for as long as 5–7 h. The procedure did not appear to disturb the normal locomotion, feeding, drinking and sleeping behavior of the animals. Plasma luteinizing hormone (LH) profiles obtained from the ovariectomized rats showed a normal pulsatile pattern of LH secretion. The frequency and amplitude of the pulses was not affected, even when the animals were bled on 2 consecutive days. Moreover, the procedure permitted accurate detection and characterization of pulsatile LH release in intact, immature rats although the episodes of LH secretion were more variable and much less pronounced. It is believed that this novel sampling technique could prove to be a valuable tool in the study of pulsatile hormone secretion in small laboratory rodents.