Sir

Caroline Herzenberg (Nature 414, 843; 2001) is correct to suggest in Correspondence that the two women in the photograph of Ivan Petrovich Pavlov and his group are Maria Kapitonovna Petrova and Maria Nikolaevna Erofeeva (alternatively spelt “Nikolayevna Yerofeyeva”). The original, taken in Pavlov's Department of Physiology at the Military Medical Academy in 1912, includes Pavlov, seventeen male co-workers and the two female co-workers.

Erofeeva and Petrova were members of the first generation of women to enter Pavlov's laboratory, capitalizing on the expanded opportunities for women in the wake of the 1905 revolution. Erofeeva performed important experiments in 1910–12 that supported Pavlov's view that any environmental agent could, in principle, become a conditional stimulus. She defended her doctoral thesis in 1912 and, unlike most of Pavlov's co-workers, continued to work in his laboratory until taking a position at the Petropavlovsk Hospital and the P. F. Lesgaft scientific institute, where she continued her research until her death in 1925.

Petrova completed her doctoral thesis, on the nature and interaction of excitation and inhibition, in 1914. She and Pavlov became lovers that same year, Petrova becoming Pavlov's most important co-worker from about 1921 until his death in 1936. She created and was the principal force behind the laboratory's increasingly important line of investigation into “the experimental pathology and therapeutics of higher nervous activity”. She directed the department of physiology and pathophysiology of higher nervous activity at the Leningrad Institute for the Improvement of Physicians from 1935 to 1941, and was also a prolific researcher and laboratory director at the Physiological Institute of the USSR Academy of Sciences. She survived the siege of Leningrad, won a Stalin prize for scientific research in 1946, and died in 1948.

Pavlov himself conducted very few of the experiments underlying his Nobel Prize-winning research on digestion and his more famous investigations on conditional reflexes, so his co-workers were central to his laboratory's research — a subject I have previously covered in Pavlov's Physiology Factory: Experiment, Interpretation, Laboratory Enterprise (Johns Hopkins University Press, Baltimore and London, 2002). I hope to rescue Erofeeva, Petrova and other equally interesting co-workers of the later period from obscurity in my forthcoming biography of Pavlov.