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We examined 24-hour melatonin rhythms from 20 patients with seasonal affective disorder (SAD) and 20 healthy volunteers. Patients and controls were individually matched for age, sex, and month of study. Plasma samples were taken at hourly intervals, and were assayed for melatonin by radio-immunoassay. The 24-hourly melatonin estimations for each individual were fitted to a cosine curve, and the significance of the curve fits was calculated. Two analyses were performed. In analysis 1 the following were calculated: (a) cosine fit, (b) significance of fits, (c) mean amplitude and acrophase (peak) and (d) mean melatonin levels. The curve fits were highly significant for all but three subjects (two patients, one control), but there were no significant differences in any measure between the two groups. In analysis 2 the comparisons were repeated and restricted to the 18 patients and 19 controls in whom there was a significantly significant melatonin rhythm. Again there were no significant differences between groups. These results suggest that the circadian rhythm of melatonin is not abnormal in SAD, and that the therapeutic effect of light in SAD is not mediated by phase shifts in melatonin secretion.

MUSCLE viscosity influences the response to a stimulus in two ways, by opposing rapid shortening or development of tension, and by retarding relaxation. The importance of the former is illustrated by the large reduction in viscosity undergone by the retractor penis of the dog when stimulated with adrenaline [Winton, 1930]: the muscle is changed from a condition in which it is capable of resisting external forces passively and economically, to one in which it can pull relatively powerfully and quickly. The importance of a high viscosity in slowing the relaxation, on the other hand, is due to the consequence that a fused contraction is obtained in plain muscle even with a low frequency of stimulation; the increase of oxygen consumption required to maintain a tetanus might, therefore, be too low to be estimated. Bayliss [1928] showed that this' consideration met the objections to the tetanus theory of tonus which had been based on failure to detect an increase of oxygen consumption during tonic contraction. The tetanus theory encounters, however, another difficulty, in that a frequency of stimulus high enough to produce a fused response ordinarily results in the rapid onset of fatigue in plain muscle, whereas normal tonic contraction appears to be unattended by fatigue. A stimulus applied once a minute, for example, produces fatigue with relatively little fusion both in the retractor penis of the dog [Win ton, 1926, Fig. 1] and in the anterior retractor of Mytilus ( Fig. 1 B below). Stimulation at a frequency high enough to produce a fused contraction induces fatigue even sooner. If, however, it were found that not all kinds of stimuli reduce the viscosity of plain muscle, but that

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