The initial biodistribution patterns of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA), an extracellular fluid contrast agent, and human serum albumin, paramagnetically labeled with 19 Gd-DTPA groups and used as an intravascular agent, were compared in the brain, heart, liver, and major mediastinal vessels of rats. Repeated 4 s spin echo images acquired after injection of 0.2 mmol/kg Gd-DTPA demonstrated a maximum enhancement between 15 and 25 s of 57% in brain, 307% in heart, 220% in liver, 83% in subcutaneous tissue, and 380% in slowly flowing blood in mediastinal vascular structures. In the following 55 s there was a continuous decrease (average 45%) in signal intensity in each tissue except brain. Albumin-(Gd-DTPA), injected at a four times lower molar dose (0.045 mmol/kg) with respect to Gd-DTPA, demonstrated maximal enhancement of brain by 34%, heart by 237%, liver by 186%, and blood in mediastinal vessels by 325%. Gadolinium-DTPA, which rapidly diffuses from the small vessels into the interstitial space, was noted to accumulate in solid tissues and subsequently to be partially eliminated within 70 s of administration. Signal enhancement achieved with albumin-(Gd-DTPA) remained at a constant level over the 70 s observation period. These data further support the notion that albumin-(Gd-DTPA), due to its predominantly intravascular distribution, might be applied advantageously for the assessment of perfusion and blood-volume disorders.