Posterior lymph heart pressure, rate and flow were measured in chronically cannulated Bufo marinus during normal hydrated and dehydrated conditions. A new surgical technique was developed which allowed direct and constant measurement of the functioning of the posterior lymph hearts with minimal disruption to normal lymph drainage. The mean intra-lymph-heart systolic pressure was 2.29 +/- 0.12 kPa for hydrated animals at rest, decreasing to 1.01 +/- 0.10 kPa after 24 h of dehydration. Similarly, lymph heart rate, which was 48.2 +/- 1.7 beats min-1 under hydrated conditions, decreased to 31.8 +/- 4.6 beats min-1 after 18 h of dehydration. Lymph flow decreased almost to zero during dehydration from a hydrated rate of 1.11 +/- 0.04 ml h-1 100 g-1. This is the first study to measure directly and to correlate these variables in an amphibian and to show specifically that pressure, rate and lymph flow are significantly reduced during periods of dehydration.