The pineal organ of the red kangaroo (Megaleia rufa) is shown to be distinguishable from that of the grey kangaroo (Macropus canguru) by being symmetrically bilobate instead of rounded distally. Pineal weights in the two species were similar. At least in M. rufa pineal size appeared to increase with age - from 6.4-15.6 mg in animals aged about 1 yr to 40.1 mg in a female 6+ years old. In comparison with the nocturnal indole contents of the pineal in rats, those of kangaroos have similar levels of 5- methoxy-N-acetyltryptamine (melatonin) and 5-hydroxyindole-3-acetic acid to the nocturnal values recorded in the literature for rats, but a lower level of 5-hydroxytryptamine (serotonin). Available information on vertebrate pineal weights and indoles does not provide evidence for any obviously distinctive evolutionary or functional status for the marsupial pineal organ as compared with that of some eutherian mammals or of some birds and reptiles.