Heart rates were monitored in the pebble crab, Gaetice depressus using a non-invasive, computer aided cardiac monitoring system. A high degree of intra- and interindividual variability was observed, as was the presence of endogenous circatidal and circadian rhythms. Both acute and sublethal toxicities of mercury were determined. LC₅₀´s (96) for mercury were between 0.16-0.20 mg l^-1. Exposure to HgCl₂ above LC₅₀ (96) (0.3 mg Hg l^-1) resulted in rapid and statictically significant increases in heart rate whereas exposure to a concentration lower than LC₅₀ (96) (0.1 mg Hg l^-1) resulted in progressive reduction in heart rate similar to that of control crabs.