Single muscle fibres from Balanus nubilus were injected with the metallochromic Ca2+ indicator arsenazo III; multi-wavelength microspectrophotometry was used to monitor changes in the absorption spectrum of such fibres during isometric tension transients elicited by electrical stimulation. Differential absorbance changes, recorded in the 660-720 nm region of the spectrum, were characterized by rapid, nearly linear increases during constant current depolarizing pulses (25-200 ms); upon termination of electrical stimuli the increased differential absorbance described an exponential (k = 0.6-2.0 s-1) decay toward baseline levels. The maximal increases in these absorbance signals preceded the peaks of the tension transients by 300-400 ms at 15-17 degrees C. Calibration of these optical transients indicated that the initial velocity of the rising phase corresponded to a 0.03 microM ms-1 increase in sarcoplasmic free Ca2+. Appreciable tension was not generated until the apparent peak magnitude of the free Ca2+ transient exceeded 2 microM; a further three-fold increase (from 2 to 6 micrometers) in the maximal value of the free Ca2+ transient was accompanied by a 20-fold increase in the magnitude of the tension transient.