In this issue, two articles present major advances in the quantitative analysis of the molecular mechanisms underlying rod phototransduction. One is by Nikonov et al. (1998), the other by Calvert et al. (1998). These two papers are complimentary, but with substantial areas of intersection. At the present time, the activation cascade in rod phototransduction that leads to the hydrolysis of the internal transmitter, cyclic GMP (cGMP) and to the closure of light-sensitive channels is fairly well understood. The inactivation steps responsible for the termination of the photoresponse and the feedback mechanisms, which modulate sensitivity and kinetics and also contribute to response termination, are not understood nearly as well. The field of phototransduction has always been fraught with controversy: for every point, there has been a counterpoint. However, one can argue, with little fear of inciting controversy, that a complete understanding of phototransduction must include an understanding of the steps by which the photoresponse is initiated and the steps by which it is terminated. For this reason, the Nikonov et al. paper, “The Kinetics of Recovery of the Dark-adapted Salamander Rod Photoresponse” is especially significant. This paper moves us closer to a definitive answer to an old, controversial question: What is the rate-limiting biochemical reaction that determines the time course of recovery of the photocurrent from a flash bright enough to temporarily shut off all light-sensitive current? Two main contenders have been the inactivation of rhodopsin and the inactivation of the activated phosphodiesterase–G-protein complex. The authors present a case for the latter. A highlight of the paper is a new approach to quantify the extent of guanylyl cyclase activation in a feedback pathway mediated by calcium. The role of cyclase in determining the time at which photocurrent recovery begins and its role in sculpting the waveform of recovery are quantified. This analysis supports the existence of at least one more significant target for calcium feedback. A notable feature of the paper is that the authors make stunning progress largely through powerful new theoretical analysis applied to data gathered with stateof-the-art techniques. A number of important observations and conclusions are stated in a formal manner in mathematical language, which includes theorems, lemmas, and proofs. The rigorous approach in this paper has the advantages of clarity and completeness. The aspects of phototransduction that can now be well understood are highlighted by a mathematical model, and the gaps in our knowledge are set off in stark contrast. Fortunately for the reader with less mathematical background, sufficient explanatory discussion surrounds the mathematical statements, such that a reader may even choose to skip the theorems entirely without major loss of information.